netfs: Reserve netfs_sreq_source 0 as unset/unknown

Reserve the 0-valued netfs_sreq_source to mean unset or unknown so that it
can be seen in the trace as such rather than appearing as
download-from-server when it's going to get switched to something else.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-9-dhowells@redhat.com/ # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/netfs.h

@@ -43,6 +43,7 @@ static inline void folio_start_private_2(struct folio *folio)
 #define NETFS_BUF_PAGECACHE_MARK XA_MARK_1	/* - Page needs wb/dirty flag wrangling */
 
 enum netfs_io_source {
+	NETFS_SOURCE_UNKNOWN,
 	NETFS_FILL_WITH_ZEROES,
 	NETFS_DOWNLOAD_FROM_SERVER,
 	NETFS_READ_FROM_CACHE,

include/trace/events/netfs.h

@@ -60,6 +60,7 @@
 	E_(netfs_rreq_trace_write_done,		"WR-DONE")
 
 #define netfs_sreq_sources					\
+	EM(NETFS_SOURCE_UNKNOWN,		"----")		\
 	EM(NETFS_FILL_WITH_ZEROES,		"ZERO")		\
 	EM(NETFS_DOWNLOAD_FROM_SERVER,		"DOWN")		\
 	EM(NETFS_READ_FROM_CACHE,		"READ")		\