[fix] get rid of checking for absent device name in vfs_get_tree()

It has no business being there, it's checked by relevant ->get_tree()
as it is *and* it returns the wrong error for no reason whatsoever.

Fixes: f3a09c92 "introduce fs_context methods"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>