6363233 zfs dmu zfetch code misuses rw locks

[roch.bourbonn...@sun.com]

Author: rbourbon
Repository: /hg/onnv/onnv-gate
Revision: c0cffb22a17829f86b8804c4aaa7e4e3b82015db
Log message:
6363233 zfs dmu zfetch code misuses rw locks

Files:
        update: usr/src/uts/common/fs/zfs/dmu_zfetch.c