rtc: pcf2127: fix a kmemleak caused in pcf2127_i2c_gather_write

pcf2127_i2c_gather_write() allocates memory as local variable
for i2c_master_send(), after finishing the master transfer,
the allocated memory should be freed. The kmemleak is reported:

unreferenced object 0xffff80231e7dba80 (size 64):
  comm "hwclock", pid 27762, jiffies 4296880075 (age 356.944s)
  hex dump (first 32 bytes):
    03 00 12 03 19 02 11 13 00 80 98 18 00 00 ff ff ................
    00 50 00 00 00 00 00 00 02 00 00 00 00 00 00 00 .P..............
  backtrace:
    [<ffff000008221398>] create_object+0xf8/0x278
    [<ffff000008a96264>] kmemleak_alloc+0x74/0xa0
    [<ffff00000821070c>] __kmalloc+0x1ac/0x348
    [<ffff0000087ed1dc>] pcf2127_i2c_gather_write+0x54/0xf8
    [<ffff0000085fd9d4>] _regmap_raw_write+0x464/0x850
    [<ffff0000085fe3f4>] regmap_bulk_write+0x1a4/0x348
    [<ffff0000087ed32c>] pcf2127_rtc_set_time+0xac/0xe8
    [<ffff0000087eaad8>] rtc_set_time+0x80/0x138
    [<ffff0000087ebfb0>] rtc_dev_ioctl+0x398/0x610
    [<ffff00000823f2c0>] do_vfs_ioctl+0xb0/0x848
    [<ffff00000823fae4>] SyS_ioctl+0x8c/0xa8
    [<ffff000008083ac0>] el0_svc_naked+0x34/0x38
    [<ffffffffffffffff>] 0xffffffffffffffff
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-pcf2127.c

@@ -303,6 +303,9 @@ static int pcf2127_i2c_gather_write(void *context,
 	memcpy(buf + 1, val, val_size);
 
 	ret = i2c_master_send(client, buf, val_size + 1);
+
+	kfree(buf);
+
 	if (ret != val_size + 1)
 		return ret < 0 ? ret : -EIO;