rbd: bench-write should return error if io-size >= 4G

rbd: bench-write should return error if io-size >= 4G

Currently if user perform bench-write with io-size > 4G
then its crashing because currently during memory allocation
bufferptr taking size of buffer as a unsigned and io-size > 4G
will overflow with unsigned. so during memset operation it will
try to set io_size size of memory area pointed by bufferptr,
(bufferptr area is:  (4G - io_size)), so it will cause
segmentation fault.

Fix is to return error if io-size >= 4G

Fixes: http://tracker.ceph.com/issues/18422
Reported-by:  Jason Dillaman <dillaman@redhat.com>
Signed-off-by: Gaurav Kumar Garg <garg.gaurav52@gmail.com>