svcrdma: fix error handling of rdma_alloc_frmr()

ib_alloc_fast_reg_mr() and ib_alloc_fast_reg_page_list() returns
ERR_PTR() and not NULL. Compile tested only.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>