[XFS] Always reset btree cursor after an insert

After a btree insert operation a cursor can be invalid due to block splits
and a maybe a new root block. We reset the cursor in xfs_bmbt_insert() in
the cases where we think we need to but it isn't enough as we still see
assertions. Just do what we do elsewhere and reset the cursor
unconditionally. Also remove the fix to revalidate the original cursor in
xfs_bmbt_insert().

SGI-PV: 983336

SGI-Modid: xfs-linux-melb:xfs-kern:31342a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>