fs: add an enum for number of life time hints

Add WRITE_LIFE_HINT_NR into the rw_hint enum to define the number of
values write life time hints can be set to. This is useful for e.g.
file systems which may want to map these values to allocation groups.

Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: yhr

include/linux/rw_hint.h

@@ -14,6 +14,7 @@ enum rw_hint {
 	WRITE_LIFE_MEDIUM	= RWH_WRITE_LIFE_MEDIUM,
 	WRITE_LIFE_LONG		= RWH_WRITE_LIFE_LONG,
 	WRITE_LIFE_EXTREME	= RWH_WRITE_LIFE_EXTREME,
+	WRITE_LIFE_HINT_NR,
 } __packed;
 
 /* Sparse ignores __packed annotations on enums, hence the #ifndef below. */