exportfs: Mark struct export_operations functions at kernel-doc

exportfs: Mark struct export_operations functions at kernel-doc

Adding a `@` before the function names make then recognizable as
kernel-docs, so they get correctly rendered in the documentation.

Even if they are already marked with `@` in the short one-line summary,
the kernel-docs will correctly favor the more detailed definition here.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260112-tonyk-fs_uuid-v1-2-acc1889de772@igalia.com
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>