exportfs,nfsd: rework checking for layout-based block device access support

exportfs,nfsd: rework checking for layout-based block device access support

Currently NFSD hard codes checking support for block-style layouts.
Lift the checks into a file system-helper and provide a exportfs-level
helper to implement the typical checks.

This prepares for supporting block layout export of multiple devices
per file system.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260423181854.743150-5-cel@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>