filesystems: return ENOSYS for REQ_PEEK

The filesystems already implement REQ_PEEK, but do not fully
use the new filesystem cache code yet. (Because it isn't committed
yet..) REQ_PEEK should be disabled for them until they do.

This indicates to VFS that they are not annotating their cache
blocks (in VM) with inode number/offset info, and therefore mmap()
shouldn't succeed on any of their files. (Most importantly exec()
won't fallback elegantly otherwise.)

Change-Id: Ic57ee422864b4bbc031eadba32973270907b02fd

servers/ext2/table.c

@@ -46,5 +46,11 @@ int (*fs_call_vec[])(void) = {
     fs_rdlink,          /* 30  */
     fs_getdents,        /* 31  */
     fs_statvfs,		/* 32  */
+#if 0
     fs_readwrite,       /* 33  */
+    no_sys,
+#else
+    no_sys,
+    no_sys,
+#endif
 };

servers/iso9660fs/table.c

@@ -41,5 +41,11 @@ int (*fs_call_vec[])(void) = {
   no_sys,			/* 30: not used */
   fs_getdents,			/* 31 */
   fs_statvfs,			/* 32 */
-  fs_read			/* 33 */
+#if 0
+  fs_read,			/* 33 */
+  no_sys,
+#else
+  no_sys,
+  no_sys,
+#endif
 };

servers/mfs/table.c

@@ -44,6 +44,6 @@ int (*fs_call_vec[])(void) = {
         fs_rdlink,	    /* 30  */
         fs_getdents,	    /* 31  */
         fs_statvfs,         /* 32  */
-        fs_readwrite,       /* 33  */
+        no_sys,             /* 33  */		/* peek not supported */
 };