Change NFS readdir() to only ignore cookies preceding the given offset for

UFS rather than for all but ZFS.  This code was assuming that offsets were
monotonically increasing for all file systems except ZFS and that the
cookies from a previous call may have been rewound to a block boundary.
According to mckusick@ only UFS is known to do this, so only requests against
UFS file systems should remove cookies smaller than the given offset.  This
fixes serving TMPFS over NFS as it too does not have monotonically increasing
offsets.  The comment around the code also indicated it was specific to UFS.

Some of the code using 'not_zfs' is specific to ZFS snapshot handling, so
add a 'is_zfs' variable for those cases.

It's possible that 'is_zfs' check for VFS_VGET() support may not be
specific to ZFS.  This needs more research and testing.

After this fix TMPFS and other file systems can be served over NFS.

To test I compared the results of syncing a /usr/src tree into a tmpfs and
serving that over NFS.  Before the fix 3589 files were missing on the remote
view.  After the fix all files were successfully found.

Reviewed by:	rmacklem
Discussed with:	mckusick, rmacklem via fs@
Discussed at:	http://lists.freebsd.org/pipermail/freebsd-fs/2014-April/019264.html
MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division

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