gfs2: check for live vs. read-only file system in gfs2_fitrim

Before this patch, gfs2_fitrim was not properly checking for a "live" file
system. If the file system had something to trim and the file system
was read-only (or spectator) it would start the trim, but when it starts
the transaction, gfs2_trans_begin returns -EROFS (read-only file system)
and it errors out. However, if the file system was already trimmed so
there's no work to do, it never called gfs2_trans_begin. That code is
bypassed so it never returns the error. Instead, it returns a good
return code with 0 work. All this makes for inconsistent behavior:
The same fstrim command can return -EROFS in one case and 0 in another.
This tripped up xfstests generic/537 which reports the error as:

    +fstrim with unrecovered metadata just ate your filesystem

This patch adds a check for a "live" (iow, active journal, iow, RW)
file system, and if not, returns the error properly.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/rgrp.c

@@ -1370,6 +1370,9 @@ int gfs2_fitrim(struct file *filp, void __user *argp)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!test_bit(SDF_JOURNAL_LIVE, &sdp->sd_flags))
+		return -EROFS;
+
 	if (!blk_queue_discard(q))
 		return -EOPNOTSUPP;