fs: fix an infinite loop in iomap_fiemap

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: ffff88...