switch do_new_mount_fc() to fc_mount()

switch do_new_mount_fc() to fc_mount()

Prior to the call of do_new_mount_fc() the caller has just done successful
vfs_get_tree().  Then do_new_mount_fc() does several checks on resulting
superblock, and either does fc_drop_locked() and returns an error or
proceeds to unlock the superblock and call vfs_create_mount().

The thing is, there's no reason to delay that unlock + vfs_create_mount() -
the tests do not rely upon the state of ->s_umount and
	fc_drop_locked()
	put_fs_context()
is equivalent to
	unlock ->s_umount
	put_fs_context()

Doing vfs_create_mount() before the checks allows us to move vfs_get_tree()
from caller to do_new_mount_fc() and collapse it with vfs_create_mount()
into an fc_mount() call.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>