MFC r290191 (by avg):

MFC r290191 (by avg):
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>

git-svn-id: svn://svn.freebsd.org/base/stable/10@290766 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f