bpf: classify block device hooks appropriately

bpf: classify block device hooks appropriately

A bunch of new hooks for managing block devices were added a while ago
but they weren't actually appropriately classified.

* bpf_lsm_bdev_alloc() is called when the inode for the block
  device is allocated. This happens from a sleepable context so mark the
  function as sleepable. When this function is called the memory for the
  block device storage embedded into the inode is zeroed. That block
  device cannot be meaningfully reference or interacted with at this
  point. So mark it as untrusted for now.

* bpf_lsm_bdev_free() is called when the inode for the block
  device is freed. A bunch of memory associated with the block device
  has already been freed and there's dangling pointers in there. So mark
  it as untrusted. It cannot be meaningfully referenced or interacted
  with anymore. It is also called from sb->s_op->free_inode:: which
  means it runs in rcu context (most of the times). So leave it as
  non-sleepable.

* bpf_lsm_bdev_setintegrity() is called when a dm-verity device
  is instantiated (glossing over details for simplicity of the commit
  message). The block device is very much alive so it remains a trusted
  hook. It's also called with device mapper's suspend lock held and so
  the hook is able to sleep so mark it sleepable.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20260326-work-bpf-bdev-v2-1-5e3c58963987@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>