dcache: Reduce the scope of i_lock in d_splice_alias

i_lock is only needed until __d_find_any_alias calls dget on the alias
dentry.  After that the reference to new ensures that dentry_kill and
d_delete will not remove the inode from the dentry, and remove the
dentry from the inode->d_entry list.

The inode i_lock came to be held over the the __d_move calls in
d_splice_alias through a series of introduction of locks with
increasing smaller scope.  First it was the dcache_lock, then
it was the dcache_inode_lock, and finally inode->i_lock.

Furthermore inode->i_lock is not held over any other calls
to d_move or __d_move so it can not provide any meaningful
rename protection.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>