UBIFS: return sensible error codes

When mounting/re-mounting, UBIFS returns EINVAL even if the ENOSPC
or EROFS codes are are much better, just because we have not found
references to ENOSPC/EROFS in mount (2) man pages. This patch
changes this behaviour and makes UBIFS return real error code,
because:

1. It is just less confusing and more logical
2. mount is not described in SuSv3, so it seems to be not really
   well-standartized
3. we do not cover all cases, and any random undocumented in man
   pages error code may be returned anyway

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/master.c b/fs/ubifs/master.c
index 71d5493..a88f338 100644
--- a/fs/ubifs/master.c
+++ b/fs/ubifs/master.c
@@ -354,7 +354,7 @@
 	int err, lnum, offs, len;
 
 	if (c->ro_media)
-		return -EINVAL;
+		return -EROFS;
 
 	lnum = UBIFS_MST_LNUM;
 	offs = c->mst_offs + c->mst_node_alsz;