vfio/mlx5: Rewrite create mkey flow to allow better code reuse

vfio/mlx5: Rewrite create mkey flow to allow better code reuse

Change the creation of mkey to be performed in multiple steps:
data allocation, DMA setup and actual call to HW to create that mkey.

In this new flow, the whole input to MKEY command is saved to eliminate
the need to keep array of pointers for DMA addresses for receive list
and in the future patches for send list too.

In addition to memory size reduce and elimination of unnecessary data
movements to set MKEY input, the code is prepared for future reuse.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>