fs/vfs: allow NULL iov_base in KERNEL build with zero-based text

This commit fixes a regression introduced by
89df084b0e. That commit
added a check to ensure iov_base is not NULL, assuming NULL
always represents an inaccessible address.

However, in CONFIG_BUILD_KERNEL mode where CONFIG_ARCH_TEXT_VBASE
is set to 0, address zero is a valid virtual address for the
user-space text segment. The previous check caused libelf to
fail with -EFAULT when attempting to load ELF program headers
into the base of the user address space.

This patch wraps the safety check in a conditional to ensure it is
only executed when address zero is not considered a valid
executable base.

Signed-off-by: Lwazi Dube <lwazeh@gmail.com>

fs/vfs/fs_read.c

@@ -178,6 +178,7 @@ ssize_t file_readv(FAR struct file *filep,
 
   /* Are all iov_base accessible? */
 
+#if !defined(CONFIG_BUILD_KERNEL) || CONFIG_ARCH_TEXT_VBASE != 0
   for (ret = 0; ret < iovcnt; ret++)
     {
       if (iov[ret].iov_base == NULL && iov[ret].iov_len != 0)
@@ -185,6 +186,7 @@ ssize_t file_readv(FAR struct file *filep,
           return -EFAULT;
         }
     }
+#endif
 
   ret = -EBADF;