f2fs: fix to relocate check condition in f2fs_fallocate()

compress and pinfile flag should be checked after inode lock held to
avoid race condition, fix it.

Fixes: 4c8ff709 ("f2fs: support data compression")
Fixes: 5fed0be8 ("f2fs: do not allow partial truncation on pinned file")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>