stop using igrab in xfs_vn_link

->link is guranteed to get an already reference inode passed so we
can do a simple increment of i_count instead of using igrab and thus
avoid banging on the global inode_lock.  This is what most filesystems
already do.

Also move the increment after the call to xfs_link to simplify error
handling.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>