gfs2: Simplify DLM_LKF_QUECVT use

The DLM_LKF_QUECVT flag needs to be set for "upward" lock conversions to
ensure fairness, but setting it for "downward" lock conversions will
lead to a failure.  The flag is currently set based on the GLF_BLOCKING
flag and it's not immediately obvious why this is correct.  Simplify
things by figuring out if a lock conversion is "upward" by looking at
the before and after locking modes instead of relying on the
GLF_BLOCKING flag.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>