fs: add fcntl() interface for setting/getting write life time hints

fs: add fcntl() interface for setting/getting write life time hints

We have a pwritev2(2) interface based on passing in flags. Add an
fcntl interface for querying these flags, and also for setting them
as well:

F_GET_WRITE_LIFE	Returns one of the valid type of write hints,
			like WRITE_HINT_MEDIUM.

F_SET_WRITE_LIFE	Pass in a WRITE_HINT_* type to set the
			write life time hint for this file/inode.
			Returns 0 on succes, -1 otherwise.

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