mke2fs support for BLKDISCARD

mke2fs support for BLKDISCARD

Try calling the BLKDISCARD ioctl at mkfs time to pre-discard all blocks
on an ssd, or a thinly-provisioned storage device.

No real error checking; if it fails, it fails, and that's ok - it's
just an optimization.  Also, it cannot work in conjunction with
the undo io manager, for obvious reasons.

Optionally disabled with a "-K" (mnemonic: Keep) option.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>