MFC r277419:

MFC r277419:
Allow skipping dmu_buf_will_dirty() call in dsl_dir_transfer_space().

dsl_dir_transfer_space() is mostly called after dsl_dir_diduse_space(),
which already calls dmu_buf_will_dirty() for the same dbuf and tx, so
its duplicate call in those cases will change nothing, only spend time.

Skipping this call by four times reduces time spent in dbuf_write_done()
and descendants, updating dataset statistics with several congested lock
acquisitions.  When rewriting 8K zvol blocks at 1GB/s rate, this reduces
CPU time spent inside dbuf_write_done(), according to profiling, from 45%
of 683K samples to 18% of 422K.

git-svn-id: svn://svn.freebsd.org/base/stable/10@278142 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f