libxfs: remove kernel stubs from xfs_shared.h

The kernel stubs added to xfs_shared.h don't belong there, and are
mostly unnecessary with the #ifdef __KERNEL__ bits added to the
xfs_ag.[ch] files. Move the one remaining needed stub in libxfs_priv.h.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>