xfs_healer: fix missing HAVE_LISTMOUNT flag and stubs

xfs_healer: fix missing HAVE_LISTMOUNT flag and stubs

xfs/666 regressed with a broken xfs_healer_start that can't actually
walk the mount namespace with listmount and statmount.  The new stubs
are wrapped with HAVE_LISTMOUNT, but this cflag is not added in healer/
Makefile even if the build system has HAVE_LISTMOUNT set.

Also fix the stubs to return error correctly -- the wrappers follow C
library behavior.

Fixes: b30356f25335eb ("libfrog: add fallback stubs for libfrog_statmount and fstatmount")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>