9p: fix cache/debug options printing in v9fs_show_options

commit 4eb3117 changed the cache= option to accept either string
shortcuts or bitfield values. It also changed /proc/mounts to emit the
option as the hexadecimal numeric value rather than the shortcut string.

However, by printing "cache=%x" without the leading 0x, shortcuts such
as "cache=loose" will emit "cache=f" and 'f' is not a string that is
parseable by kstrtoint(), so remounting may fail if a remount with
"cache=f" is attempted.

debug=%x has had the same problem since options have been displayed in
c4fac91 ("9p: Implement show_options")

Fix these by adding the 0x prefix to the hexadecimal value shown in
/proc/mounts.

Fixes: 4eb3117 ("fs/9p: Rework cache modes and add new options to Documentation")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Message-ID: <54b93378-dcf1-4b04-922d-c8b4393da299@redhat.com>
[Dominique: use %#x at Al Viro's suggestion, also handle debug]
Tested-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: Eric Sandeen

fs/9p/v9fs.c

@@ -132,7 +132,7 @@ int v9fs_show_options(struct seq_file *m, struct dentry *root)
 	struct v9fs_session_info *v9ses = root->d_sb->s_fs_info;
 
 	if (v9ses->debug)
-		seq_printf(m, ",debug=%x", v9ses->debug);
+		seq_printf(m, ",debug=%#x", v9ses->debug);
 	if (!uid_eq(v9ses->dfltuid, V9FS_DEFUID))
 		seq_printf(m, ",dfltuid=%u",
 			   from_kuid_munged(&init_user_ns, v9ses->dfltuid));
@@ -148,7 +148,7 @@ int v9fs_show_options(struct seq_file *m, struct dentry *root)
 	if (v9ses->nodev)
 		seq_puts(m, ",nodevmap");
 	if (v9ses->cache)
-		seq_printf(m, ",cache=%x", v9ses->cache);
+		seq_printf(m, ",cache=%#x", v9ses->cache);
 #ifdef CONFIG_9P_FSCACHE
 	if (v9ses->cachetag && (v9ses->cache & CACHE_FSCACHE))
 		seq_printf(m, ",cachetag=%s", v9ses->cachetag);