btrfs: get rid of 'holder' propagation

In all cases it (as well as info->bdev_holder) is equal to
&btrfs_root_fs_type.  And all it's ever used for is
blkdev_get_by_path() - the rest is propagation of that
constant to (all) calls of that function in fs/btrfs.

Just pass the right value in those 4 calls and to hell with
storing it/passing it around/etc.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>