[PATCH] prevent do_brk() from allocating below mmap_min_addr

[PATCH] prevent do_brk() from allocating below mmap_min_addr

This is the 2.4 equivalent of the following 2.6 patch by Eric Paris :
ecaf18c15aac8bb9bed7b7aa0e382fe252e275d5

Given a specifically crafted binary do_brk() can be used to get low
pages available in userspace virtually memory and can thus be used to
circumvent the mmap_min_addr low memory protection.  Add security checks
in do_brk().

Signed-off-by: Willy Tarreau <w@1wt.eu>