client: trim deleted inode

client: trim deleted inode

Previous patch makes MDS send notification to clients when an inode
is deleted. When receiving a such notification, we invalidate any
dentry link to the deleted inode. If there is no other reference to
the inode, the inode gets trimmed.

For cephfs fuse client, we use fuse_lowlevel_notify_inval_entry() or
fuse_lowlevel_notify_delete() to notify the kernel to trim the deleted
inode. (this is not completely reliable because we play unlink/link
tricks when  handle MDS replies. it's difficult to keep the user space
cache and kernel dcache in sync)

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