This patch removes BKL protection from the invocation of the
super_operations ->statfs() method, and shifts it into the filesystems
where necessary. Any out-of-tree filesystems may need to take the BKL in
their statfs() methods if they were relying on it for synchronisation.

All ->statfs() implementations have been modified to take the BKL,
except for those that don't reference any external mutable data or that
already have their own locking.

Additionally, capifs is changed to use simple_statfs rather than its
own home-grown version.

The BKL change has been flagged at the end of
Documentation/filesystems/porting, along with the recent change to
->permission BKL usage.