bluestore: print aio in batch

bluestore: print aio in batch

KernelDevice::aio_{submit,write,read}() are critical paths. calling
cct->_conf->subsys.should_gather() multi-times is not optimal. the
downside of this issue is that if the aio is printed, the size of
buffer in PrebufferedStreambuf could be large if the number of iov is
large, that could be a heavy load to the memory subsystem.

Signed-off-by: Kefu Chai <kchai@redhat.com>