This uses the sg_pcopy_from_buffer to copy data, instead of doing it
ourselves.

In addition to reducing code size, this fixes the following oops
resulting from failing to kmap the page:

[   68.896381] Unable to handle kernel NULL pointer dereference at virtual address 00000ab8
[   68.904539] pgd = 3561adb3
[   68.907475] [00000ab8] *pgd=00000000
[   68.911153] Internal error: Oops: 805 [#1] ARM
[   68.915618] Modules linked in: cfg80211 rfkill des_generic libdes arc4 libarc4 cbc ecb algif_skcipher sha256_generic libsha256 sha1_generic hmac aes_generic libaes cmac sha512_generic md5 md4 algif_hash af_alg i2c_imx i2c_core ci_hdrc_imx ci_hdrc mxs_dcp ulpi roles udc_core imx_sdma usbmisc_imx usb_common firmware_class virt_dma phy_mxs_usb nf_tables nfnetlink ip_tables x_tables ipv6 autofs4
[   68.950741] CPU: 0 PID: 139 Comm: mxs_dcp_chan/ae Not tainted 5.10.34 #296
[   68.958501] Hardware name: Freescale i.MX6 Ultralite (Device Tree)
[   68.964710] PC is at memcpy+0xa8/0x330
[   68.968479] LR is at 0xd7b2bc9d
[   68.971638] pc : [<c053e7c8>]    lr : [<d7b2bc9d>]    psr: 000f0013
[   68.977920] sp : c2cbbee4  ip : 00000010  fp : 00000010
[   68.983159] r10: 00000000  r9 : c3283a40  r8 : 1a5a6f08
[   68.988402] r7 : 4bfe0ecc  r6 : 76d8a220  r5 : c32f9050  r4 : 00000001
[   68.994945] r3 : 00000ab8  r2 : fffffff0  r1 : c32f9050  r0 : 00000ab8
[   69.001492] Flags: nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[   69.008646] Control: 10c53c7d  Table: 83664059  DAC: 00000051
[   69.014414] Process mxs_dcp_chan/ae (pid: 139, stack limit = 0x667b57ab)
[   69.021133] Stack: (0xc2cbbee4 to 0xc2cbc000)
[   69.025519] bee0:          c32f9050 c3235408 00000010 00000010 00000ab8 00000001 bf10406c
[   69.033720] bf00: 00000000 00000000 00000010 00000000 c32355d0 832fb080 00000000 c13de2fc
[   69.041921] bf20: c3628010 00000010 c33d5780 00000ab8 bf1067e8 00000002 c21e5010 c2cba000
[   69.050125] bf40: c32f8040 00000000 bf106a40 c32f9040 c3283a80 00000001 bf105240 c3234040
[   69.058327] bf60: ffffe000 c3204100 c2c69800 c2cba000 00000000 bf103b84 00000000 c2eddc54
[   69.066530] bf80: c3204144 c0140d1c c2cba000 c2c69800 c0140be8 00000000 00000000 00000000
[   69.074730] bfa0: 00000000 00000000 00000000 c0100114 00000000 00000000 00000000 00000000
[   69.082932] bfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[   69.091131] bfe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
[   69.099364] [<c053e7c8>] (memcpy) from [<bf10406c>] (dcp_chan_thread_aes+0x4e8/0x840 [mxs_dcp])
[   69.108117] [<bf10406c>] (dcp_chan_thread_aes [mxs_dcp]) from [<c0140d1c>] (kthread+0x134/0x160)
[   69.116941] [<c0140d1c>] (kthread) from [<c0100114>] (ret_from_fork+0x14/0x20)
[   69.124178] Exception stack(0xc2cbbfb0 to 0xc2cbbff8)
[   69.129250] bfa0:                                     00000000 00000000 00000000 00000000
[   69.137450] bfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[   69.145648] bfe0: 00000000 00000000 00000000 00000000 00000013 00000000
[   69.152289] Code: e320f000 e4803004 e4804004 e4805004 (e4806004)
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>