invalidate_mapping_pages(): add cond_resched

invalidate_mapping_pages() can sometimes take a long time (millions of pages
to free).  Long enough for the softlockup detector to trigger.

We used to have a cond_resched() in there but I took it out because the
drop_caches code calls invalidate_mapping_pages() under inode_lock.

The patch adds a nasty flag and puts the cond_resched() back.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>