Reserve feature bit and SB field bit for filename encoding

Reserve feature bit and SB field bit for filename encoding

The s_encoding field in the superblock stores a magic number indicating
the encoding format and version used globally by file and directory
names in the filesystem.

The s_encoding_flags defines policies for using the charset encoding,
like how to handle invalid sequences and what kind of normalization to
use.

A feature flag is also allocated to indicate whether this filesystem has
encoding awareness enabled.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>