r228827 fixed a problem where copying of NFSv4 open credentials into

r228827 fixed a problem where copying of NFSv4 open credentials into
a credential structure would corrupt it. This happened when the
p argument was != NULL. However, I now realize that the copying of
open credentials should only happen for p == NULL, since that indicates
that it is a read-ahead or write-behind. This patch fixes this.
After this commit, r228827 could be reverted, but I think the code is
clearer and safer with the patch, so I am going to leave it in.
Without this patch, it was possible that a NFSv4 VOP_SETATTR() could have
changed the credentials of the caller. This would have happened if
the process doing the VOP_SETATTR() did not have the file open, but
some other process running as a different uid had the file open for writing
at the same time.

MFC after:	5 days