btrfs: handle FS_IOC_READ_VERITY_METADATA ioctl

Commit 14605409 ("btrfs: initial fsverity support") introduced
fs-verity support for btrfs, but didn't add support for
FS_IOC_READ_VERITY_METADATA to directly query the Merkle tree,
descriptor and signature blocks for fs-verity enabled files.

Add the (trival) implementation: we just need to wire it through to the
fs-verity code, the same way as is done in the other two filesystems
which support this ioctl (ext4, f2fs). The fs-verity code already has
access to the required data.

This is also safe to backport to older stable trees (5.15+) if needed.

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>