IB/fmr_pool: Allocate page list for pool FMRs only when caching enabled

Allocate memory for the page_list field of struct ib_pool_fmr only
when caching is enabled for the FMR pool, since the field is not used
otherwise.  This can save significant amounts of memory for large
pools with caching turned off.

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