nvmet: Introduce nvmet_req_transfer_len()

Add the new function nvmet_req_transfer_len() to parse a request command
to extract the transfer length of the command. This function
implementation relies on multiple helper functions for parsing I/O
commands (nvmet_io_cmd_transfer_len()), admin commands
(nvmet_admin_cmd_data_len()) and fabrics connect commands
(nvmet_connect_cmd_data_len).

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>