Now we introduce iget_locked and iget5_locked. These are similar to
iget, but return a locked inode and read_inode has not been called. So
the FS has to call read_inode to initialize the inode and then unlock
it with unlock_new_inode().

This patch is based on the icreate patch from the XFS group, i.e.
it is pretty much identical except for function naming.