NFSD: Refactor NFSv4 OPEN(CREATE)

[ Upstream commit 254454a5 ]

Copy do_nfsd_create() to nfs4proc.c and remove NFSv3-specific logic.

[ cel: backported to 5.10.y, prior to idmapped mounts ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>