when get fiemap starting from MAX_LFS_FILESIZE, (maxbytes - *len) < start
will always true , then *len set zero. because of start offset is beyond
file size, for erofs filesystem it will always return iomap.length with
zero,iomap iterate will enter infinite loop. it is necessary cover this
corner case to avoid this situation.

------------[ cut here ]------------
WARNING: CPU: 7 PID: 905 at fs/iomap/iter.c:35 iomap_iter+0x97f/0xc70
Modules linked in: xfs erofs
CPU: 7 PID: 905 Comm: iomap Tainted: G        W         5.17.0-rc8 #27
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
RIP: 0010:iomap_iter+0x97f/0xc70
Code: 85 a1 fc ff ff e8 71 be 9c ff 0f 1f 44 00 00 e9 92 fc ff ff e8 62 be 9c ff 0f 0b b8 fb ff ff ff e9 fc f8 ff ff e8 51 be 9c ff <0f> 0b e9 2b fc ff ff e8 45 be 9c ff 0f 0b e9 e1 fb ff ff e8 39 be
RSP: 0018:ffff888060a37ab0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff888060a37bb0 RCX: 0000000000000000
RDX: ffff88807e19a900 RSI: ffffffff81a7da7f RDI: ffff888060a37be0
RBP: 7fffffffffffffff R08: 0000000000000000 R09: ffff888060a37c20
R10: ffff888060a37c67 R11: ffffed100c146f8c R12: 7fffffffffffffff
R13: 0000000000000000 R14: ffff888060a37bd8 R15: ffff888060a37c20
FS:  00007fd3cca01540(0000) GS:ffff888108780000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020010820 CR3: 0000000054b92000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 iomap_fiemap+0x1c9/0x2f0
 erofs_fiemap+0x64/0x90 [erofs]
 do_vfs_ioctl+0x40d/0x12e0
 __x64_sys_ioctl+0xaa/0x1c0
 do_syscall_64+0x35/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
 </TASK>
---[ end trace 0000000000000000 ]---
watchdog: BUG: soft lockup - CPU#7 stuck for 26s! [iomap:905]
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Guo Xuenan <guoxuenan@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[djwong: fix some typos]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>