Bluestore,NVMEDEVICE: fix the I/O logic for READ

Bluestore,NVMEDEVICE: fix the I/O logic for READ

Aio_submit will submit both aio_read/write, and also there
are synchronized read and random read, so we need to
handle the read I/O completion in a correct way.

Since random read has its own ioc, so the
num_reading for ioc will be at most 1, which will be easy
to handle in io_complete. And we need only to differentiate
whethere it is an aio_read.

Also fix the exception logic in command send, make the style
consistent.

Signed-off-by: optimistyzy <optimistyzy@gmail.com>