Use invalidate_lock instead of fuse's private i_mmap_sem. The intended
purpose is exactly the same. By this conversion we fix a long standing
race between hole punching and read(2) / readahead(2) paths that can
lead to stale page cache contents.

CC: Miklos Szeredi <miklos@szeredi.hu>
Reviewed-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NJan Kara <jack@suse.cz>