NFSD: Move documenting comment for nfsd4_process_open2()

[ Upstream commit 7e2ce0cc ]

Clean up nfsd4_open() by converting a large comment at the only
call site for nfsd4_process_open2() to a kerneldoc comment in
front of that function.

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