sbin/hammer: Move cache to tailq end when get_buffer_data() has a valid cache

sbin/hammer: Move cache to tailq end when get_buffer_data() has a valid cache

This is the same as what get_buffer() does when new allocation
is not needed. This hasn't been done in get_buffer_data() when
the given non NULL buffer is the one to look for.

This seems to make newfs_hammer a bit faster, as the i/o size is
normally large enough to invoke eviction of HAMMER userspace buffer
cache while running, starting from the head of the tailq.

-- before this commit
 # for x in 1 2 3 4 5; do
 > (time newfs_hammer -L TEST /dev/da1 /dev/da2 /dev/da3 > /dev/null) 2>&1 | grep real
 > done
 real    0m20.291s
 real    0m20.574s
 real    0m22.347s
 real    0m21.562s
 real    0m20.991s

-- with this commit
 # for x in 1 2 3 4 5; do
 > (time newfs_hammer -L TEST /dev/da1 /dev/da2 /dev/da3 > /dev/null) 2>&1 | grep real
 > done
 real    0m18.023s
 real    0m18.064s
 real    0m18.104s
 real    0m17.646s
 real    0m18.310s