os/WBThrottle: added check of return value for posix_fadvise()

check the return value of posix_fadvise() in WBThrottle::entry()

Signed-off-by: Xing Lin <xinglin@cs.utah.edu>
rgw/rgw_op: added check of return value for rgw_unlink_bucket()

checked the return value of rgw_unlink_bucket() in RGWCreateBucket::execute()

Signed-off-by: Xing Lin <xinglin@cs.utah.edu>