ceph: pre-allocate ceph_cap struct for ceph_add_cap()

So that ceph_add_cap() can be used while i_ceph_lock is locked.
This simplifies the code that handle cap import/export.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>

diff --cc fs/ceph/caps.c
Simple merge

diff --cc fs/ceph/inode.c
Simple merge

diff --cc fs/ceph/super.h
Simple merge