ash: [MEMALLOC] Add pushstackmark

ash: [MEMALLOC] Add pushstackmark

Upstream commit:

    Author: Herbert Xu <herbert@gondor.apana.org.au>
    Date:   Sat Oct 6 00:45:52 2007 +0800

    [MEMALLOC] Add pushstackmark

    This patch gets rid of the stack mark tracking hack by allocating a little
    bit of stack memory if we're at risk of planting a stack mark which may be
    grown later.  To do this a new function pushstackmark is added which lets
    the user pick a bigger amount to allocate since some users do that anyway
    after setting a stack mark.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>