[XFS] split xfs_icsb_balance_counter

Add an xfs_icsb_balance_counter_locked for the case where mp->m_sb_lock is
already locked.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30918a

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