IB: Enable FMR pool user to set page size

This patch allows the consumer to set the page size of "pages" mapped
by the pool FMRs, which is a feature already existing in the base
verbs API.  On the cosmetic side it changes ib_fmr_attr.page_size field
to be named page_shift.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>