ublk: fix maple tree lockdep warning in ublk_buf_cleanup

ublk: fix maple tree lockdep warning in ublk_buf_cleanup

ublk_buf_cleanup() iterates the maple tree with mas_for_each()
without holding mas_lock, triggering a lockdep splat on
CONFIG_PROVE_RCU kernels since mas_find() internally uses
rcu_dereference_check() which requires either RCU or the tree lock.

Fix by holding mas_lock around the iteration, and call mas_erase()
before freeing each range to avoid dangling pointers in the tree.

Fixes: 5e864438e285 ("ublk: replace xarray with IDA for shmem buffer index allocation")
Reported-by: Jens Axboe <axboe@kernel.dk>
Closes: https://lore.kernel.org/linux-block/0349d72d-dff8-4f9f-b448-919fa5ae96da@kernel.dk/
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260423033058.2805135-2-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>