configure: always check for statmount supported_mask

configure: always check for statmount supported_mask

On systems with kernel headers older than v6.8 that preceed the addition
of the statmount syscall, have_listmount will be set to false and thus
checking of the statmount supported_mask is being skipped since the
check is currently conditional to listmount support.

This causes compilation failures since need_internal_statmount will not
be set, and thus struct statmount will not be available to xfs_healer.

Fix this by always checking for statmount supported_mask support even
when listmount is not available.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>