Message type and related cleanup

- Intorduce and use a message type for VFS_GETDENTS, VFS_READ,
   VFS_WRITE.

 - Some cleanup to related functions where vir_bytes are replaced (and
   casted to/from, in parameter definition and local variables as well.

   This allow to see more clearly which function receives unsafe
   (pointer) values, or at least values which are not supposed to be
   valid in the address space of VFS. The current patch does so only
   for the minimal amount of functions which are concerned with the
   introduction of the new message type.

Change-Id: I0cdca97409c4016d02fae067b48bf55d37572c5c

include/minix/callnr.h

@@ -232,9 +232,4 @@
 
 #define NR_VFS_CALLS		49	/* highest number from base plus one */
 
-/* Field names for the read(2), write(2), and getdents(2) calls. */
-#define VFS_READWRITE_FD	m1_i1	/* int */
-#define VFS_READWRITE_BUF	m1_p1	/* char * */
-#define VFS_READWRITE_LEN	m1_i2	/* size_t */
-
 #endif /* !_MINIX_CALLNR_H */

include/minix/ipc.h

@@ -225,7 +225,7 @@ _ASSERT_MSG_SIZE(mess_lc_vfs_getvfsstat);
 typedef struct {
 	int fd;
 	unsigned long req;
-	void *arg;
+	vir_bytes arg;
 
 	uint8_t padding[44];
 } mess_lc_vfs_ioctl;
@@ -312,6 +312,15 @@ typedef struct {
 } mess_lc_vfs_readlink;
 _ASSERT_MSG_SIZE(mess_lc_vfs_readlink);
 
+typedef struct {
+	int fd;
+	vir_bytes buf;
+	size_t len;
+
+	uint8_t padding[44];
+} mess_lc_vfs_readwrite;
+_ASSERT_MSG_SIZE(mess_lc_vfs_readwrite);
+
 typedef struct {
 	uint32_t nfds;
 	fd_set *readfds;
@@ -866,6 +875,7 @@ typedef struct {
 		mess_lc_vfs_path	m_lc_vfs_path;
 		mess_lc_vfs_pipe2	m_lc_vfs_pipe2;
 		mess_lc_vfs_readlink	m_lc_vfs_readlink;
+		mess_lc_vfs_readwrite	m_lc_vfs_readwrite;
 		mess_lc_vfs_select	m_lc_vfs_select;
 		mess_lc_vfs_stat	m_lc_vfs_stat;
 		mess_lc_vfs_statvfs1	m_lc_vfs_statvfs1;