UBIFS: re-arrange variables in ubifs_info

This is a cosmetic patch which re-arranges variables in 'struct ubifs_info'
so that all boolean-like variables which are only changed during mounting or
re-mounting to R/W mode are places together. Then they are turned into
bit-fields, which makes the structure a little bit smaller.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>