[XFS] kill usesless IHOLD calls in xfs_rename

Similar to to the previous patch for remove and rmdir only grab a
reference to inodes when we join them to transaction to balance the
decrement on transaction completion. Everything else it taken care of by
the VFS.

Note that the old case had leaks of inode count when src == target or src
or target == one of the parent inodes, but these cases are fortunately
already rejected by the VFS.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30904a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>