io_uring: Add 'write_mode' option for optional cmds

io_uring: Add 'write_mode' option for optional cmds

Add a new option 'write_mode' to support additional optional Write
command family such as Write Uncorrectable and Write Zeroes in NVMe.
Since we have io_uring_cmd ioengine where we can define the actual
opcode of the command, this option will be used to test NVMe device with
various combination of Write command types.

'write_mode' option can be given either 'write', 'uncor', 'zeroes' or
'verify'.  'write' is for normal Write command which is by default,
'uncor' is for Write Uncorrectable, 'zeroes' for Write Zeroes and 'verify'
for Verify command  This should be used with DDIR_WRITE ddir.

This patch updates command's opcode in fio_ioring_init() to avoid
branches in the I/O hottest path giving opcode value to the
fio_nvme_uring_cmd_prep() as an argument.

Signed-off-by: Minwoo Im <minwoo.im@samsung.com>