NFSv4.1: Bump the default callback session slot count to 16

Users can still control this value explicitly using the
max_session_cb_slots module parameter, but let's bump the default
up to 16 for now.
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4session.h

@@ -10,7 +10,7 @@
 
 /* maximum number of slots to use */
 #define NFS4_DEF_SLOT_TABLE_SIZE (64U)
-#define NFS4_DEF_CB_SLOT_TABLE_SIZE (1U)
+#define NFS4_DEF_CB_SLOT_TABLE_SIZE (16U)
 #define NFS4_MAX_SLOT_TABLE (1024U)
 #define NFS4_NO_SLOT ((u32)-1)