test/zdtm: Fix test_msg massive stack usage

In test_msg() a buffer is allocated on stack to cook the outputed message.
This buffer's size was defined using the PAGE_SIZE constant defined in
zdtmtst.h file.

On some system like ppc64, the page size is large (64K), leading to massive
stack allocation, which may be too large in case of alternate stack like
the one used in the sigaltstack test.

This fix, defines a 2048 characters buffer for test_msg, and expose a
constant to allocate stack accordingly in the sigaltstack test.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>