fix the crap in dst/dcore
fix the crap in dst/dcore

* don't reinvent the wheels, please - open_bdev_exclusive() is there
  for purpose
* both open_by_devnum() and open_bdev_exclusive() return ERR_PTR(...)
  upon error, not NULL

Signed-off-by: Al Viro <de609eb4d5d70b1d38ec6642adbfc33a2781f63c@zeniv.linux.org.uk>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas