e2fsprogs: Add discard function into struct_io_manager

e2fsprogs: Add discard function into struct_io_manager

In order to provide generic "discard" function for all e2fsprogs tools
add a discard function prototype into struct_io_manager. Specific
function for specific io managers can be crated that way.

This commit also creates unix_discard function which uses BLKDISCARD
ioctl to discard data blocks on the block device and bind it into
unit_io_manager structure to be available for all e2fsprogs tools.
Note that BLKDISCARD is still Linux specific ioctl, however other
unix systems may provide similar functionality. So far the
unix_discard() remains linux specific hence is embedded in #ifdef
__linux__ macro.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>