f2fs: change to use atomic_t type form sbi.atomic_files

inode_lock[ATOMIC_FILE] was used for protecting sbi->atomic_files,
update atomic_files variable's type to atomic_t instead of unsigned
int, then inode_lock[ATOMIC_FILE] can be obsoleted.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>