RDMA/uverbs: Update outdated reference to remove_commit_idr_uobject()

RDMA/uverbs: Update outdated reference to remove_commit_idr_uobject()

The function remove_commit_idr_uobject() was split into
destroy_hw_idr_uobject() and remove_handle_idr_uobject() by
commit 0f50d88a6e9a ("IB/uverbs: Allow all DESTROY commands
to succeed after disassociate").  The kref put that the
comment refers to now lives in remove_handle_idr_uobject().
Update the stale reference.

Also update "allocated this IDR with a NULL object" to
"allocated this XArray entry with a NULL pointer" to match
the actual data structure (xa_store) and the wording already
used two lines below ("transfers our kref on uobj to the
XArray").

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Link: https://patch.msgid.link/20260321105859.7642-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Leon Romanovsky <leon@kernel.org>