Btrfs: make sure compressed bios don't complete too soon

When writing a compressed extent, a number of bios are created that
point to a single struct compressed_bio.  At end_io time an atomic counter in
the compressed_bio struct makes sure that all of the bios have finished
before final end_io processing is done.

But when multiple bios are needed to write a compressed extent, the
counter was being incremented after the first bio was sent to submit_bio.
It is possible the bio will complete before the counter is incremented,
making the end_io handler free the compressed_bio struct before
processing is finished.

The fix is to increment the atomic counter before bio submission,
both for compressed reads and writes.

Signed-off-by: Chris Mason <chris.mason@oracle.com>