IB/rdmavt: Check lkey_table_size value before use

The lkey_table_size driver specific parameter value is used before its
value is sanity checked and restricted to RVT_MAX_LKEY_TABLE_BITS.

This causes a vmalloc allocation failure for large values. Fix this
by moving the value check before the first usage of the value.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>