ublk: refactor common helper ublk_shmem_remove_ranges()

ublk: refactor common helper ublk_shmem_remove_ranges()

Extract the shared walk+erase+unpin+kfree loop into
ublk_shmem_remove_ranges(). When buf_index >= 0, only ranges matching
that index are removed; when buf_index < 0, all ranges are removed.

Also extract ublk_unpin_range_pages() to share the page unpinning
loop.

Convert both __ublk_ctrl_unreg_buf() and ublk_buf_cleanup() to use
the new helper.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260423033058.2805135-3-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>