Disable negative name caching for msdosfs to work around a bug.

Since the name cache is case-sensitive and msdosfs isn't,
creating a file 'foo' won't invalidate a negative entry for 'FOO'.
There are similar problems related to 8.3 filenames.

A better solution is to override VOP_LOOKUP with a method that
canonicalizes the name, then calls vfs_cache_lookup().  Unfortunately,
it's not quite that simple because vfs_cache_lookup() will call
msdosfs_lookup() on a cache miss, and msdosfs_lookup() needs a way to
get at the original component name.

sys/fs/msdosfs/msdosfs_lookup.c

@@ -380,11 +380,18 @@ notfound:
 		cnp->cn_flags |= SAVENAME;
 		return (EJUSTRETURN);
 	}
+#if 0
 	/*
 	 * Insert name into cache (as non-existent) if appropriate.
+	 *
+	 * XXX Negative caching is broken for msdosfs because the name
+	 * cache doesn't understand peculiarities such as case insensitivity
+	 * and 8.3 filenames.  Hence, it may not invalidate all negative
+	 * entries if a file with this name is later created.
 	 */
 	if ((cnp->cn_flags & MAKEENTRY) && nameiop != CREATE)
 		cache_enter(vdp, *vpp, cnp);
+#endif
 	return (ENOENT);
 
 found: