fs/nfs: Use enum req_op where appropriate

Improve static type checking by using enum req_op for request operations.
Rename an 'rw' argument into 'op' since that name is typically used for
request operations. This patch does not change any functionality. Note:
REQ_OP_READ = READ = 0 and REQ_OP_WRITE = WRITE = 1.

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-58-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>