fat: add config option to set UTF-8 mount option by default

FAT has long supported its own default file name encoding config
setting, separate from CONFIG_NLS_DEFAULT.

However, if UTF-8 encoded file names are desired FAT character set
should not be set to utf8 since this would make file names case
sensitive even if case insensitive matching is requested.  Instead,
"utf8" mount options should be provided to enable UTF-8 file names in
FAT file system.

Unfortunately, there was no possibility to set the default value of this
option so on UTF-8 system "utf8" mount option had to be added manually
to most FAT mounts.

This patch adds config option to set such default value.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/vfat.txt

@@ -56,9 +56,10 @@ iocharset=<name> -- Character set to use for converting between the
 		 you should consider the following option instead.
 
 utf8=<bool>   -- UTF-8 is the filesystem safe version of Unicode that
-		 is used by the console.  It can be enabled for the
-		 filesystem with this option. If 'uni_xlate' gets set,
-		 UTF-8 gets disabled.
+		 is used by the console. It can be enabled or disabled
+		 for the filesystem with this option.
+		 If 'uni_xlate' gets set, UTF-8 gets disabled.
+		 By default, FAT_DEFAULT_UTF8 setting is used.
 
 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
 		 escaped sequences.  This would let you backup and