nvmet-tcp: validate so_priority modparam value

The module parameter so_priority is passed to the function
sock_set_priority() which has following prototype and expect
priotity arg type to be u32:-

void sock_set_priority(struct sock *sk, u32 priority);

Add a module parameter validation callback to reject any negative
values for the so_priority as it is defigned as int. Use this
oppurtunity to update the module parameter description and print the
default value.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>