libceph: create source file "net/ceph/snapshot.c"

This creates a new source file "net/ceph/snapshot.c" to contain
utility routines related to ceph snapshot contexts.  The main
motivation was to define ceph_create_snap_context() as a common way
to create these structures, but I've moved the definitions of
ceph_get_snap_context() and ceph_put_snap_context() there too.
(The benefit of inlining those is very small, and I'd rather
keep this collection of functions together.)

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/net/ceph/Makefile b/net/ceph/Makefile
index e87ef43..958d9856 100644
--- a/net/ceph/Makefile
+++ b/net/ceph/Makefile
@@ -11,5 +11,5 @@
 	crypto.o armor.o \
 	auth_x.o \
 	ceph_fs.o ceph_strings.o ceph_hash.o \
-	pagevec.o
+	pagevec.o snapshot.o