vfs: create a generic checking and prep function for FS_IOC_SETFLAGS

Create a generic function to check incoming FS_IOC_SETFLAGS flag values
and later prepare the inode for updates so that we can standardize the
implementations that follow ext4's flag values.

Note that the efivarfs implementation no longer fails a no-op SETFLAGS
without CAP_LINUX_IMMUTABLE since that's the behavior in ext*.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>

include/linux/fs.h

@@ -3546,4 +3546,7 @@ static inline struct sock *io_uring_get_socket(struct file *file)
 }
 #endif
 
+int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
+			     unsigned int flags);
+
 #endif /* _LINUX_FS_H */