ublk: eliminate permanent pages[] array from struct ublk_buf

ublk: eliminate permanent pages[] array from struct ublk_buf

The pages[] array (kvmalloc'd, 8 bytes per page = 2MB for a 1GB buffer)
was stored permanently in struct ublk_buf but only needed during
pin_user_pages_fast() and maple tree construction. Since the maple tree
already stores PFN ranges via ublk_buf_range, struct page pointers can
be recovered via pfn_to_page() during unregistration.

Make pages[] a temporary allocation in ublk_ctrl_reg_buf(), freed
immediately after the maple tree is built. Rewrite __ublk_ctrl_unreg_buf()
to iterate the maple tree for matching buf_index entries, recovering
struct page pointers via pfn_to_page() and unpinning in batches of 32.
Simplify ublk_buf_erase_ranges() to iterate the maple tree by buf_index
instead of walking the now-removed pages[] array.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-5-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>