Fix an underflow bug in __lmb_alloc_base

__lmb_alloc_base can underflow if it fails to find free space.  This was fixed
in linux with commit d9024df02ffe74d723d97d552f86de3b34beb8cc.  This patch
merely updates __lmb_alloc_base to resemble the current version in Linux.

Signed-off-by: Andy Fleming <afleming@freescale.com>