BKL: Explicitly add BKL around get_sb/fill_super

This patch is a preparation necessary to remove the BKL from do_new_mount().
It explicitly adds calls to lock_kernel()/unlock_kernel() around
get_sb/fill_super operations for filesystems that still uses the BKL.

I've read through all the code formerly covered by the BKL inside
do_kern_mount() and have satisfied myself that it doesn't need the BKL
any more.

do_kern_mount() is already called without the BKL when mounting the rootfs
and in nfsctl. do_kern_mount() calls vfs_kern_mount(), which is called
from various places without BKL: simple_pin_fs(), nfs_do_clone_mount()
through nfs_follow_mountpoint(), afs_mntpt_do_automount() through
afs_mntpt_follow_link(). Both later functions are actually the filesystems
follow_link inode operation. vfs_kern_mount() is calling the specified
get_sb function and lets the filesystem do its job by calling the given
fill_super function.

Therefore I think it is safe to push down the BKL from the VFS to the
low-level filesystems get_sb/fill_super operation.

[arnd: do not add the BKL to those file systems that already
       don't use it elsewhere]

Signed-off-by: Jan Blunck <jblunck@infradead.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Christoph Hellwig <hch@infradead.org>

fs/freevxfs/vxfs_super.c

@@ -148,7 +148,7 @@ static int vxfs_remount(struct super_block *sb, int *flags, char *data)
  *   The superblock on success, else %NULL.
  *
  * Locking:
- *   We are under the bkl and @sbp->s_lock.
+ *   We are under @sbp->s_lock.
  */
 static int vxfs_fill_super(struct super_block *sbp, void *dp, int silent)
 {
@@ -159,11 +159,14 @@ static int vxfs_fill_super(struct super_block *sbp, void *dp, int silent)
 	struct inode *root;
 	int ret = -EINVAL;
 
+	lock_kernel();
+
 	sbp->s_flags |= MS_RDONLY;
 
 	infp = kzalloc(sizeof(*infp), GFP_KERNEL);
 	if (!infp) {
 		printk(KERN_WARNING "vxfs: unable to allocate incore superblock\n");
+		unlock_kernel();
 		return -ENOMEM;
 	}
 
@@ -236,6 +239,7 @@ static int vxfs_fill_super(struct super_block *sbp, void *dp, int silent)
 		goto out_free_ilist;
 	}
 
+	unlock_kernel();
 	return 0;
 	
 out_free_ilist:
@@ -245,6 +249,7 @@ out_free_ilist:
 out:
 	brelse(bp);
 	kfree(infp);
+	unlock_kernel();
 	return ret;
 }