rpma: add support for File System DAX

rpma: add support for File System DAX

File System DAX is handled in a different way than Device DAX:

1) In case of File System DAX, each thread uses a separate file
from this file system and no offset is needed. In case of Device DAX,
each thread uses a separate offset within the same Device DAX.

2) File System DAX requires rpma_mr_advise(3)(ibv_advise_mr(3))
to be called for the registered memory to avoid page faults
and degraded performance.

Ref: https://github.com/axboe/fio/issues/1238

Signed-off-by: Wang, Long <long1.wang@intel.com>