Patch from Hugh Dickins <hugh@veritas.com>

When prune_icache coincides with unmounting, invalidate_inodes notices
the inode it's working on as busy but doesn't wait: Self-destruct in 5
seconds message, and later iput oopses on freed super_block.

Neither end is a fast path, so the patch just adds iprune_sem for exclusion.

The semaphore is held across dispose_list so that
dispose_list->clear_inode->destroy_inode cannot reference a destroyed
superblock.