Memory mappings were only truncated on an explicit truncate, but not when the
file size was changed externally.

Fix this by moving the truncation code from fuse_setattr to
fuse_change_attributes.

Yes, there are races between write and and external truncation, but we can't
really do anything about them.
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>