f2fs: fix to update map->m_next_extent correctly in f2fs_map_blocks()

[ Upstream commit 869833f5 ]

Script to reproduce:
mkfs.f2fs -O extra_attr,compression /dev/vdb -f
mount /dev/vdb /mnt/f2fs -o mode=lfs,noextent_cache
cd /mnt/f2fs
f2fs_io write 1 0 1024 rand dsync testfile
xfs_io testfile -c "fsync"
f2fs_io write 1 0 512 rand dsync testfile
xfs_io testfile -c "fsync"
cd /
umount /mnt/f2fs
mount /dev/vdb /mnt/f2fs
f2fs_io precache_extents /mnt/f2fs/testfile
umount /mnt/f2fs

Tracepoint output:
f2fs_update_read_extent_tree_range: dev = (253,16), ino = 4, pgofs = 0, len = 512, blkaddr = 1055744, c_len = 0
f2fs_update_read_extent_tree_range: dev = (253,16), ino = 4, pgofs = 513, len = 351, blkaddr = 17921, c_len = 0
f2fs_update_read_extent_tree_range: dev = (253,16), ino = 4, pgofs = 864, len = 160, blkaddr = 18272, c_len = 0

During precache_extents, there is off-by-one issue, we should update
map->m_next_extent to pgofs rather than pgofs + 1, if last blkaddr is
valid and not contiguous to previous extent.

Fixes: c4020b2d ("f2fs: support F2FS_IOC_PRECACHE_EXTENTS")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>