mlx4_core: Support creation of FMRs with pages smaller than 4K

Don't hard code a test against a minimum page shift of 12, since the
device may support smaller pages.  Test against the actual smallest
page size from the device capabilities.

Signed-off-by: Oren Duer <oren@mellanox.co.il>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>