There's no user of huge_valid_dev() any more, so remove it.

No functional change.
Signed-off-by: NYaowei Bai <bywxiaobai@163.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>