MFV r272803:

Illumos issue:
    5175 implement dmu_read_uio_dbuf() to improve cached read performance

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272809 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f