xfsdump: Initialize getbmap structure in quantity2offset

Prevent uninitialized data in the stack by initializing getbmap structure
to zero.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>