l2arc: do not call trim_map_free() for blocks with zero b_asize

b_asize can be zero if the block is compressed into an empty block
(ZIO_COMPRESS_EMPTY) and the trim code asserts that meaningless
zero-sized trimming is not attempted.
The logic for calling trim_map_free() is extracted into a new function
l2arc_trim() to minimize code duplication.

PR: 203473
Reported by: Willem Jan Withagen <wjw@digiware.nl>
Tested by: Willem Jan Withagen <wjw@digiware.nl>
MFC after: 11 days