usnic: fix module_recv_buffers perf regression

Cisco v1.6 git commit 913ec6c and upstream trunk r29593 (segfault fix)
introduced a performance regression by inadvertently disabling the
`module_recv_buffers` functionality.  With those changes in place, the
`btl_usnic_recv.c` logic would end up mallocing a buffer that should
have otherwise come from a `module_recv_buffers` pool.  It also resulted
in a small, bounded memory leak (128 buffers at each power-of-two size
interval).

The new version just places the buffer after the free list item with a
flexible array member.  I bumped the pool to allocate all 128 elements
up front because the deferred allocation was modestly impacting IMB
Sendrecv performance at a few sizes.

Reviewed-by: Reese Faucette <rfaucett@cisco.com>