block: test-iosched: Restrict number of bios in the test request

num_bios is passed in test_iosched_add_wr_rd_test_req().
Restrict the number of bios to bios buffer size while preparing test
request. Otherwise, it may lead to the buffer overflow if num_bios is
larger than the size of bios buffer.

Change-Id: Ib523f2b5bd91fea7b24f3b1260ebdcb1a1b7b9ab
Signed-off-by: Siba Prasad <sibap@codeaurora.org>