btrfs: get rid of re-entering of btrfs_get_tree()

[EXISTING PROBLEM]
Currently btrfs mount is split into two parts:

- btrfs_get_tree_subvol()
  Which sets up the very basic fs_info, and eventually calls
  mount_subvol() to mount the target subvolume.

- btrfs_get_tree_super()
  This is the part doing super block allocation and if there is no
  existing super block, do the real open_ctree() to open the fs.

However currently we're doing this in a complex re-entering way:

vfs_get_tree()
|- btrfs_get_tree()
   |- btrfs_get_tree_subvol()
      |- vfs_get_tree()
      |  |- btrfs_get_tree()
      |     |- btrfs_get_tree_super()
      |- mount_subvol()

This is definitely not that easy to grasp.

[ENHANCEMENT]
The function vfs_get_tree() is only doing the following work:

- Call get_tree() call back
- Call super_wake()
- Call security_sb_set_mnt_opts()

In our case, super_wake() can be skipped, as after
btrfs_get_tree_subvol() finishes, vfs_get_tree() will call super_wake()
on the super block we got anyway.

The same applies to security_sb_set_mnt_opts(), as long as we do not
free the security from our original fc in btrfs_get_tree_subvol(), the
first vfs_get_tree() call will handle the security correctly.

So here we only need to:

- Replace vfs_get_tree() call with btrfs_get_tree_super()

- Keep the existing fc->security for vfs_get_tree() to handle the
  security

This will remove the re-entering behavior and make thing much easier to
follow.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>