ext4: simplify ext4_sb_read_encoding

Return the encoding table as the return value instead of as an argument,
and don't bother with the encoding flags as the caller can handle that
trivially.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>