nfsd: minor nfsd read api cleanup

Christoph points that the NFSv2/v3 callers know which case they want
here, so we may as well just call the file=NULL case directly instead of
making this conditional.

Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --cc fs/nfsd/nfs3proc.c
Simple merge

diff --cc fs/nfsd/nfs4xdr.c
Simple merge

diff --cc fs/nfsd/nfsproc.c
Simple merge

diff --cc fs/nfsd/vfs.c
Simple merge

diff --cc fs/nfsd/vfs.h
Simple merge