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>

linux-user/mmap.c

@@ -216,6 +216,7 @@ static abi_ulong mmap_next_start = TASK_UNMAPPED_BASE;
 
 unsigned long last_brk;
 
+#ifdef CONFIG_USE_GUEST_BASE
 /* Subroutine of mmap_find_vma, used when we have pre-allocated a chunk
    of guest address space.  */
 static abi_ulong mmap_find_vma_reserved(abi_ulong start, abi_ulong size)
@@ -249,6 +250,7 @@ static abi_ulong mmap_find_vma_reserved(abi_ulong start, abi_ulong size)
     mmap_next_start = addr;
     return last_addr;
 }
+#endif
 
 /*
  * Find and reserve a free memory area of size 'size'. The search
@@ -271,9 +273,11 @@ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size)
 
     size = HOST_PAGE_ALIGN(size);
 
+#ifdef CONFIG_USE_GUEST_BASE
     if (RESERVED_VA) {
         return mmap_find_vma_reserved(start, size);
     }
+#endif
 
     addr = start;
     wrapped = repeat = 0;