btrfs: handle null fs_info in btrfs_panic()

At least backref_tree_panic() can apparently pass
in a null fs_info, so handle that in __btrfs_panic
to get the message out on the console.

The btrfs_panic macro also uses fs_info, but that's
largely pointless; it's testing to see if
BTRFS_MOUNT_PANIC_ON_FATAL_ERROR is not set.
But if it *were* set, __btrfs_panic() would have,
well, paniced and we wouldn't be here, testing it!
So just BUG() at this point.

And since we only use fs_info once now, just use it
directly.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ctree.h

@@ -3653,11 +3653,14 @@ __printf(5, 6)
 void __btrfs_panic(struct btrfs_fs_info *fs_info, const char *function,
 		   unsigned int line, int errno, const char *fmt, ...);
 
+/*
+ * If BTRFS_MOUNT_PANIC_ON_FATAL_ERROR is in mount_opt, __btrfs_panic
+ * will panic().  Otherwise we BUG() here.
+ */
 #define btrfs_panic(fs_info, errno, fmt, args...)			\
 do {									\
-	struct btrfs_fs_info *_i = (fs_info);				\
-	__btrfs_panic(_i, __func__, __LINE__, errno, fmt, ##args);	\
-	BUG_ON(!(_i->mount_opt & BTRFS_MOUNT_PANIC_ON_FATAL_ERROR));	\
+	__btrfs_panic(fs_info, __func__, __LINE__, errno, fmt, ##args);	\
+	BUG();								\
 } while (0)
 
 /* acl.c */

fs/btrfs/super.c

@@ -289,7 +289,7 @@ void __btrfs_panic(struct btrfs_fs_info *fs_info, const char *function,
 	vaf.va = &args;
 
 	errstr = btrfs_decode_error(errno, nbuf);
-	if (fs_info->mount_opt & BTRFS_MOUNT_PANIC_ON_FATAL_ERROR)
+	if (fs_info && (fs_info->mount_opt & BTRFS_MOUNT_PANIC_ON_FATAL_ERROR))
 		panic(KERN_CRIT "BTRFS panic (device %s) in %s:%d: %pV (%s)\n",
 			s_id, function, line, &vaf, errstr);