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>
diff --git a/libxfs/libxfs_priv.h b/libxfs/libxfs_priv.h
index ad920cd..b2c3f69 100644
--- a/libxfs/libxfs_priv.h
+++ b/libxfs/libxfs_priv.h
@@ -125,7 +125,7 @@
 
 #define xfs_info(mp,fmt,args...)	cmn_err(CE_CONT, _(fmt), ## args)
 #define xfs_notice(mp,fmt,args...)	cmn_err(CE_NOTE, _(fmt), ## args)
-#define xfs_warn(mp,fmt,args...)	cmn_err(CE_WARN, _(fmt), ## args)
+#define xfs_warn(mp,fmt,args...)	cmn_err((mp) ? CE_WARN : CE_WARN, _(fmt), ## args)
 #define xfs_err(mp,fmt,args...)		cmn_err(CE_ALERT, _(fmt), ## args)
 #define xfs_alert(mp,fmt,args...)	cmn_err(CE_ALERT, _(fmt), ## args)