nfsd: clear NFSEXP_PNFS flags if not supports pnfs in kernel

nfsd should not show "pnfs" flag in proc file if the filesystem does not support pnfs.

# cat /proc/fs/nfsd/exports
# Version 1.1
# Path Client(Flags) # IPs
/testfs    *(rw,insecure,no_root_squash,sync,wdelay,no_subtree_check,pnfs,fsid=0,uuid=37b07525:dc5e4203:99cc71b0:3df9dfa5,sec=1)

---
 fs/nfsd/nfs4layouts.c | 2 ++
 1 file changed, 2 insertions(+)

index 6904213..f4c9bf3 100644
@@ -125,6 +125,8 @@ void nfsd4_setup_layout_type(struct svc_export *exp)
 	    sb->s_export_op->map_blocks &&
 	    sb->s_export_op->commit_blocks)
 		exp->ex_layout_type = LAYOUT_BLOCK_VOLUME;
+	else
+		exp->ex_flags &= ~NFSEXP_PNFS;
 }
 
 static void

2.3.4

Hmm, do we want to silently clear or reject it? Either way the !CONFIG_NFSD_PNFS stub needs to do the same.

There's a note:

	/* quietly ignore unknown words and anything
	 * following. Newer user-space can try to set
	 * new values, then see what the result was.
	 */

Not sure if that's the best approach, but I guess we should stick with it.

--b.