xfs: initialize log buf I/O completion wq on log alloc

Log buffer I/O completion passes through the high priority
m_log_workqueue rather than the default metadata buffer workqueue. The
log buffer wq is initialized at I/O submission time. The log buffers are
reused once initialized, however, so this is not necessary.

Initialize the log buffer I/O completion workqueue pointers once when
the log is allocated and log buffers initialized rather than on every
log buffer I/O submission.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>