Both pinned_sb and new_sb indicate if a new superblock is needed,
so we can just remove new_sb.

Note now we must check if kernfs_tryget_sb() returns NULL, because
when it returns NULL, kernfs_mount() may still re-use an existing
superblock, which is just allocated by another concurent mount.
Suggested-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NZefan Li <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>