mds: initial code for lock cache

mds: initial code for lock cache

The lock cache preserves locks and authpins required for directory
operations. MDS can create a lock cache when it has acquired all locks
of a directory operation. The lock cache can be used to for later
operations of the same type on the same directory.

For example, when mds has acquired all locks of a unlink operation,
it creates a lock cache, which holds holds wrlocks on direcotry inode's
filelock and nestlock, rdlocks on ancestor inodes' snaplocks. For later
unlink operations on the same directory, MDS only needs to xlock the
dentry to unlink and xlock linklock of the inode to unlink.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>