f2fs: ro: compress: fix to avoid caching unaligned extent

f2fs: ro: compress: fix to avoid caching unaligned extent

Mapping info from dump.f2fs:
i_addr[0x2d] cluster flag    		[0xfffffffe : 4294967294]
i_addr[0x2e]                 		[0x   10428 : 66600]
i_addr[0x2f]                 		[0x   10429 : 66601]
i_addr[0x30]                 		[0x   1042a : 66602]

f2fs_io fiemap 37 1 /mnt/f2fs/disk-58390c8c.raw

Previsouly, it missed to align fofs and ofs_in_node to cluster_size,
result in adding incorrect read extent cache, fix it.

Before:
f2fs_update_read_extent_tree_range: dev = (253,48), ino = 5, pgofs = 37, len = 4, blkaddr = 66600, c_len = 3

After:
f2fs_update_read_extent_tree_range: dev = (253,48), ino = 5, pgofs = 36, len = 4, blkaddr = 66600, c_len = 3

Fixes: 94afd6d6e525 ("f2fs: extent cache: support unaligned extent")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>