nfs/localio: remove extra indirect nfs_to call to check {read,write}_iter

[ Upstream commit 0978e5b8 ]

Push the read_iter and write_iter availability checks down to
nfs_do_local_read and nfs_do_local_write respectively.

This eliminates a redundant nfs_to->nfsd_file_file() call.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Stable-dep-of: 992203a1 ("nfs/localio: restore creds before releasing pageio data")
Signed-off-by: Sasha Levin <sashal@kernel.org>