btrfs: simplify extent type checks in run_delalloc_nocow

There is no point in checking the type of the extent again just to set
the 'type' variable, when this check has already been performed before.
Instead, extend the original if branch with an 'else' clause. This
allows to remove one local variable and make it obvious how the code
flow differs for prealloc/regular extents.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>