[linux] Use fstat() rather than statx()

The statx() system call has a clean header file and a consistent
layout, but was unfortunately added only in kernel 4.11.

Using stat() or fstat() directly is extremely messy since glibc does
not necessarily use the kernel native data structures.  However, as
the only current use case is to obtain the length of an open file, we
can merely provide a wrapper that does precisely this.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/linux/linux_sysfs.c

@@ -40,7 +40,6 @@ FILE_LICENCE ( GPL2_OR_LATER );
  * @ret len		Length read, or negative error
  */
 int linux_sysfs_read ( const char *filename, userptr_t *data ) {
-	struct statx statx;
 	size_t offset;
 	size_t len;
 	ssize_t read;
@@ -57,13 +56,12 @@ int linux_sysfs_read ( const char *filename, userptr_t *data ) {
 	}
 
 	/* Get file length */
-	if ( linux_statx ( fd, "", AT_EMPTY_PATH, STATX_SIZE, &statx ) == -1 ) {
+	if ( linux_fstat_size ( fd, &len ) == -1 ) {
 		rc = -ELINUX ( linux_errno );
 		DBGC ( filename, "LINUX could not stat %s: %s\n",
 		       filename, linux_strerror ( linux_errno ) );
 		goto err_stat;
 	}
-	len = statx.stx_size;
 
 	/* Allocate buffer */
 	*data = umalloc ( len );