extend --use-null-mr option for server side

[bureddy]

specifying --user-null-mr option at the server side in READ/WRITE tests will perform remote memory read/writes from HCA.

[bureddy]

@flx42 FYI

[HassanKhadour]

Hi @bureddy

Thanks for your contribution!, Please add more detailed explanation in the commit message and align the man page

[sshaulnv]

@bureddy, since now its not client support only, please adapt the help section and the man page ("Allocate a null memory region for the client with ibv_alloc_null_mr.").

[sshaulnv]

feature was added: f961e40db8740011bdb55f07066cdd5fbbf7ecad