vfs: Add IOP_XATTR inode operations flag

The IOP_XATTR inode operations flag in inode->i_opflags indicates that
the inode has xattr support.  The flag is automatically set by
new_inode() on filesystems with xattr support (where sb->s_xattr is
defined), and cleared otherwise.  Filesystems can explicitly clear it
for inodes that should not have xattr support.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>