dma-buf: Rename debugfs symbols

Rename the debugfs list and mutex so it's clear they are now usable
without the need for CONFIG_DEBUG_FS. The list will always be populated
to support the creation of a BPF iterator for dmabufs.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250522230429.941193-2-tjmercier@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>