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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>