GFS2: Remove obsolete assert

Given that a resource group has been locked, there is no reason why
we should not be able to allocate as many blocks as are free. The
al_requested parameter should really be considered as a minimum
number of blocks to be available. Should this limit be overshot,
there are other mechanisms which will prevent over allocation.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>