libxfs: consume the xfs_warn mountpoint argument

Fix these warnings because xfs_warn doesn't do anything in userspace:

xfs_alloc.c: In function ‘xfs_alloc_get_rec’:
xfs_alloc.c:246:34: warning: unused variable ‘mp’ [-Wunused-variable]
  246 |         struct xfs_mount        *mp = cur->bc_mp;
      |                                  ^~

Signed-off-by: Darrick J. Wong <djwong@kernel.org>