md/bitmap: don't abuse i_writecount for bitmap files.

md bitmap code currently tries to use i_writecount to stop any other
process from writing to out bitmap file.  But that is really an abuse
and has bit-rotted so locking is all wrong.

So discard that - root should be allowed to shoot self in foot.

Still use it in a much less intrusive way to stop the same file being
used as bitmap on two different array, and apply other checks to
ensure the file is at least vaguely usable for bitmap storage
(is regular, is open for write.  Support for ->bmap is already checked
elsewhere).

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.h

@@ -605,7 +605,6 @@ extern int md_check_no_bitmap(struct mddev *mddev);
 extern int md_integrity_register(struct mddev *mddev);
 extern void md_integrity_add_rdev(struct md_rdev *rdev, struct mddev *mddev);
 extern int strict_strtoul_scaled(const char *cp, unsigned long *res, int scale);
-extern void restore_bitmap_write_access(struct file *file);
 
 extern void mddev_init(struct mddev *mddev);
 extern int md_run(struct mddev *mddev);