fs: add support for an inode to carry write hint related data

fs: add support for an inode to carry write hint related data

No functional changes in this patch, just in preparation for
allowing applications to pass in hints about data life times
for writes. Set aside 3 bits for carrying hint information
in the inode flags.

Adds the public hints as well, which are:

WRITE_LIFE_NONE		No hints about write life time
WRITE_LIFE_SHORT	Data written has a short life time
WRITE_LIFE_MEDIUM	Data written has a medium life time
WRITE_LIFE_LONG		Data written has a long life time
WRITE_LIFE_EXTREME	Data written has an extremely long life tim

Helpers are defined to store these values in flags, by passing in
the shift that's appropriate for the given use case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>