ocking, and clear it when it's not.

This prevents unnecessary blocking in readahead and (when writable mmaps are
allowed) writeback.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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