linux-user: fix compile failure if !CONFIG_USE_GUEST_BASE

linux-user: fix compile failure if !CONFIG_USE_GUEST_BASE

If CONFIG_USE_GUEST_BASE is not defined, gcc complains:
 linux-user/mmap.c:235: error: comparison of unsigned expression >= 0 is always true

because RESERVED_VA is #defined to 0. Since mmap_find_vma_reserved()
will never be called anyway if RESERVED_VA is always 0, fix this by
simply #ifdef'ing away the function and its callsite.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>