VFS: Implement REQ_BPEEK.

This commit introduces a new request type called REQ_BPEEK. It
requests minor device blocks from the FS.  Analogously to REQ_PEEK,
it requests the filesystem to get the requested blocks into its
cache, without actually copying the result anywhere.

Change-Id: If1d06645b0e17553a64b3167091e9d12efeb3d6f

lib/libvtreefs/table.c

@@ -38,6 +38,7 @@ int (*fs_call_vec[])(void) = {
 	fs_getdents,	/* 31	getdents	*/
 	fs_statvfs,	/* 32	statvfs		*/
 	no_sys,		/* 33   peek            */
+	no_sys,		/* 34   bpeek           */
 };
 
 /* This should not fail with "array size is negative": */