generic_fillattr is an optional helper that isn't used by all file
systems, move handling purely based on inode flags to vfs_getattr_nosec,
which is common code.

This fixes setting this flag for file systems not using generic_fillattr
like xfs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>