fs: reduce pointer chasing in is_mgtime() test

The is_mgtime test checks whether the FS_MGTIME flag is set in the
fstype. To get there from the inode though, we have to dereference 3
pointers.

Add a new IOP_MGTIME flag, and have inode_init_always() set that flag
when the fstype flag is set. Then, make is_mgtime test for IOP_MGTIME
instead.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241113-mgtime-v1-1-84e256980e11@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jtlayton

fs/inode.c

@@ -241,6 +241,8 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_opflags = 0;
 	if (sb->s_xattr)
 		inode->i_opflags |= IOP_XATTR;
+	if (sb->s_type->fs_flags & FS_MGTIME)
+		inode->i_opflags |= IOP_MGTIME;
 	i_uid_write(inode, 0);
 	i_gid_write(inode, 0);
 	atomic_set(&inode->i_writecount, 0);

include/linux/fs.h

@@ -623,6 +623,7 @@ is_uncached_acl(struct posix_acl *acl)
 #define IOP_NOFOLLOW	0x0004
 #define IOP_XATTR	0x0008
 #define IOP_DEFAULT_READLINK	0x0010
+#define IOP_MGTIME	0x0020
 
 /*
  * Keep mostly read-only and often accessed (especially for
@@ -2581,7 +2582,7 @@ struct file_system_type {
  */
 static inline bool is_mgtime(const struct inode *inode)
 {
-	return inode->i_sb->s_type->fs_flags & FS_MGTIME;
+	return inode->i_opflags & IOP_MGTIME;
 }
 
 extern struct dentry *mount_bdev(struct file_system_type *fs_type,