befs: check return of sb_min_blocksize

Confirm sb_min_blocksize() succeeded before continuing.

Link: http://lkml.kernel.org/r/1465700235-22881-1-git-send-email-luisbg@osg.samsung.com


Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>