NFSD: Clean up nfsd_open_verified()

[ Upstream commit f4d84c52 ]

Its only caller always passes S_IFREG as the @type parameter. As an
additional clean-up, add a kerneldoc comment.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>