linux-user: Avoid mmap of the last byte of the reserved_va

linux-user: Avoid mmap of the last byte of the reserved_va

There is an overflow problem in mmap_find_vma_reserved:
when reserved_va == UINT32_MAX, end may overflow to 0.
Rather than a larger rewrite at this time, simply avoid
the final byte of the VA, which avoids searching the
final page, which avoids the overflow.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1741
Fixes: 95059f9c ("include/exec: Change reserved_va semantics to last byte")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20230629080835.71371-1-richard.henderson@linaro.org>