os/bluestore: _txc_create inside of alloc_and_submit_lock

os/bluestore: _txc_create inside of alloc_and_submit_lock

Create the transaction inside of the SMR lock.  Otherwise, we may get a
deadlock between the cleaner C and a normal write op W:

W                         C
_txc_create seq 1
                          lock alloc_and_submit
                          _txc_create seq 2
                          ...
                          unlock alloc_and_submit
lock alloc_and_submit
...
block on flush
                          _txc_finish_io, but blocked by seq 1
<deadlock>

The root issue here is the txc's are misordered with respect to the
alloc_and_submit lock.

Fix by moving the _txc_create inside the lock!

Signed-off-by: Sage Weil <sage@newdream.net>