selftests: proc: Make va_max 1MB

Currently proc-self-map-files-002.c sets va_max (max test address
of user virtual address) to 4GB, but it is too big for 32bit
arch and 1UL << 32 is overflow on 32bit long.
Also since this value should be enough bigger than vm.mmap_min_addr
(64KB or 32KB by default), 1MB should be enough.

Make va_max 1MB unconditionally.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>