nvme: unexport functions with no external caller

There are no callers for nvme_reset_ctrl_sync() and
nvme_alloc_request_qid() so that we keep the symbols exported.

Unexport those functions, mark them static and update the header file
respectively.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>