xfs: Fix xfs_swap_extents() after removal of xfs_flushinval_pages()

Commit fb595814

 removed
xfs_flushinval_pages() and changed its callers to use
filemap_write_and_wait() and  truncate_pagecache_range() directly.

But in xfs_swap_extents() this change accidental switched the argument
for 'tip' to 'ip'. This patch switches it back to 'tip'

Signed-off-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>