fs: add support for allowing applications to pass in write life time hints

fs: add support for allowing applications to pass in write life time hints

Add four flags for the pwritev2(2) system call, allowing an application
to give the kernel a hint about what on-media life times can be
expected from a given write.

The intent is for these values to be relative to each other, no
absolute meaning should be attached to these flag names.

Define IOCB flags to carry this information over, and finally
transform them into the block defined stream values.

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