BACKPORT: 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.

Bug: 298879042
Bug: 308706939
Change-Id: I0f8c0284d9f0c05a7418474707f715dad1e404e3
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 9ed8c733)
[zhoudan8: Resolved minor conflict in fs/f2fs/debug.c and fs/f2fs/f2fs.h]
Signed-off-by: zhoudan8 <zhoudan8@xiaomi.com>