Currently in ext4_collapse_range() and ext4_punch_hole() we're
discarding preallocation twice. Once before we attempt to do any changes
and second time after we're done with the changes.
While the second call to ext4_discard_preallocations() in
ext4_punch_hole() case is not needed, we need to discard preallocation
right after ext4_ext_remove_space() in collapse range case because in
the case we had to restart a transaction in the middle of removing space
we might have new preallocations created.
Remove unneeded ext4_discard_preallocations() ext4_punch_hole() and move
it to the better place in ext4_collapse_range()