libceph: allocate dummy osdmap in ceph_osdc_init()

This leads to a simpler osdmap handling code, particularly when dealing
with pi->was_full, which is introduced in a later commit.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/osdmap.h

@@ -225,6 +225,7 @@ static inline int ceph_decode_pgid(void **p, void *end, struct ceph_pg *pgid)
 	return 0;
 }
 
+struct ceph_osdmap *ceph_osdmap_alloc(void);
 extern struct ceph_osdmap *ceph_osdmap_decode(void **p, void *end);
 struct ceph_osdmap *osdmap_apply_incremental(void **p, void *end,
 					     struct ceph_osdmap *map);