RDMA/cxgb3: Fix shift calc in build_phys_page_list() for 1-entry page lists

RDMA/cxgb3: Fix shift calc in build_phys_page_list() for 1-entry page lists

A single entry (addr 0x10001000, size 0x2000) will get converted to
page address 0x10000000 with a page size of 0x4000.  The code as it
stands doesn't address the single buffer case, but in fact it allows
the subsequent single-buffer special case to be eliminated entirely.
Because the mask now includes the (page adjusted) starting and ending
addresses, the general case works for the single buffer case as well.

Signed-off-by: Bryan Rosenburg <rosnbrg@us.ibm.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>