BKL: Remove BKL from USB gadgetfs

The BKL is only used in fill_super, which is  protected by the superblocks
s_umount rw_semaphore. Therefore it is safe to remove the BKL entirely.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: linux-usb@vger.kernel.org