libceph: factor out logic from ceph_osdc_start_request()

Factor out logic from ceph_osdc_start_request() into a new helper,
__ceph_osdc_start_request().  ceph_osdc_start_request() now amounts to
taking locks and calling __ceph_osdc_start_request().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
libceph: take map_sem for read in handle_reply()

Handling redirect replies requires both map_sem and request_mutex.
Taking map_sem unconditionally near the top of handle_reply() avoids
possible race conditions that arise from releasing request_mutex to be
able to acquire map_sem in redirect reply case.  (Lock ordering is:
map_sem, request_mutex, crush_mutex.)

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
libceph: do not dereference a NULL bio pointer

Commit f38a5181d9f3 ("ceph: Convert to immutable biovecs") introduced
a NULL pointer dereference, which broke both rbd and cephfs in -rc1.
Fix it.

Cc: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>