fs/mmap: add fd validation to correct errno

MIRTOS-111

Errors EBADF correctly. This is only a quick fix, and a lot needs
to be done with mmap().

Change-Id: Ica88360972447fbdd7c6af3bf76d8eeba4747db2
Signed-off-by: Peter Bee <bijunda1@xiaomi.com>

fs/mmap/fs_rammap.c

@@ -140,6 +140,12 @@ FAR void *rammap(int fd, size_t length, off_t offset)
 
   /* Allocate a region of memory of the specified size */
 
+  if ((unsigned int)fd >= CONFIG_NFILE_DESCRIPTORS)
+    {
+      errcode = EBADF;
+      goto errout;
+    }
+
   alloc = (FAR uint8_t *)kumm_malloc(sizeof(struct fs_rammap_s) + length);
   if (!alloc)
     {