btrfs-progs: fsck: fix a infinite loop on discount file extent repair

btrfs-progs: fsck: fix a infinite loop on discount file extent repair

For a special case, discount file extent repair function will cause
infinite loop.

The case is, if the file loses all its extents, we won't have a hole
to fill, causing repair function doing nothing. Since the
I_ERR_DISCOUNT doesn't disappear, fsck will do an infinite loop.

For such case, just puch hole to fill the whole range to fix it.

Reported-by: Robert Munteanu <robert.munteanu@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>