VFS, FSes: add REQ_PEEK request type

REQ_PEEK behaves just like REQ_READ except that it does not copy
data anywhere, just obtains the blocks from the FS into the cache.

To be used by the future mmap implementation.

Change-Id: I1b56de304f0a7152b69a72c8962d04258adb44f9

include/minix/const.h

@@ -72,6 +72,7 @@
 #define BYTE            0377	/* mask for 8 bits */
 #define READING            0	/* copy data to user */
 #define WRITING            1	/* copy data from user */
+#define PEEKING            2	/* retrieve FS data without copying */
 #define HAVE_SCATTERED_IO  1	/* scattered I/O is now standard */
 
 /* Memory is allocated in clicks. */

include/minix/vfsif.h

@@ -114,8 +114,9 @@ typedef struct {
 #define REQ_RDLINK	(VFS_BASE + 30)
 #define REQ_GETDENTS	(VFS_BASE + 31)
 #define REQ_STATVFS	(VFS_BASE + 32)
+#define REQ_PEEK	(VFS_BASE + 33)
 
-#define NREQS			    33
+#define NREQS			    34
 
 #define IS_VFS_RQ(type) (((type) & ~0xff) == VFS_BASE)