mkfs: set agsize prior to calculating minimum log size

mkfs: set agsize prior to calculating minimum log size

Each btree has its own maxlevels variable.  Since the level count of
certain btrees depend on agblocks, it's necessary to know the AG size
prior to calculating the log reservations.  These reservations are
needed to calculate the log size and the kernel will refuse to mount
if we guess too low, so stuff in the real agsize when we're formatting
the log.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>