netfs: Provide helpers to perform NETFS_RREQ_IN_PROGRESS flag wangling

Provide helpers to clear and test the NETFS_RREQ_IN_PROGRESS and to insert
the appropriate barrierage.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250701163852.2171681-4-dhowells@redhat.com


Tested-by: Steve French <sfrench@samba.org>
Reviewed-by: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>