mke2fs: If the device supports discard, don't print an error message

mke2fs: If the device supports discard, don't print an error message

Check to see if the device supports discard before starting the
progress bar, and then printing an error about inappropriate ioctl for
device (when creating a file system image to a file, for example).

Also, add a function signature in the ext2_io.h header file for
io_channel_discard() and fix an extra, uneeded argument in mke2fs's
call to that function.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>