mksquashfs: fix larger than physical memory check

Get_physical_memory() should have been returning the full amount of
memory in the system, rather than the amount of physical memory
taken by default (physical memory / SQUASHFS_TAKE).

This inadvertantly broke the "larger than physical memory" check because
it assumed get_physical_memory() returned the full amount.  In fact it
broke running Mksquashfs without -mem because the default amount of
memory taken failed the check (default amount of memory taken is
always going to be greater than or equal to the default amount of memory
taken!).

Apologies to anyone affected by the breakage.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>