hammer dedup in HEAD now has a memory limiting option

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    The hammer dedup and dedup-simulate directives in HEAD now accept
    a memory use limit option, e.g. '-m 200m'.  The default memory use
    limit is 1G.  If the dedup code exceeds the memory limit it will
    automatically restrict the CRC range it collects information on and
    will loop as many times as necessary to dedup the whole disk.

    This should make dedup viable inside qemu or other virtual
    environments.

    A few minor I/O optimizations were also made to try to pre-cache the
    b-tree metadata blocks and to allow the dedup code to get past areas
    already dedupped more quickly.  Initial dedups will still take a long
    time.

    ^C and ^T are also now supported during hammer dedup runs so you can
    see the progress.  It has to pre-scan the b-tree but once it actually
    gets into dedupping stuff ^T will give you a good indication of its
    progress.  ^C was being ignored before and now works as well.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>