mds: always create multiversion inode when cow inode

mds: always create multiversion inode when cow inode

When moving a snapshotted regular file to stray directory, we need
to create an snaprealm for it. The snaprealm is required for building
correct snapshot context when purging file objects.

Current code normally create seperate inodes when a snapshotted file
inode is COWed. Each time the file inode is COWed, the head inode's
'first' is updated. So when deleting the file, we can't use head inode's
'first' to check if it was snapshotted.

This commit make MDCache::journal_cow_dentry always create multiversion
inode. multiversion inode's 'first' keeps constant.

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