This mirrors code in tmpfs.
This changge shouldn't affect much read path, it may cause unnecessary
vm_page_lookup calls in the case where v_object has no active or inactive
pages but has some cache pages. I believe this situation to be non-essential.
In write path this change should allow us to properly detect the above
case and free a cache page when we write to a range that corresponds to it.
If this situation is undetected then we could have a discrepancy between
data in page cache and in ARC or on disk.
This change allows us to re-enable vn_has_cached_data() check in zfs_write.
NOTE: strictly speaking resident_page_count and cache fields of v_object
should be exmined under VM_OBJECT_LOCK, but for this particular usage
we may get away with it.
Discussed with: alc, kib
Approved by: pjd
Tested with: tools/regression/fsx
MFC after: 3 weeks
8a3883cfb7