ecryptfs: set s_blocksize from lower fs in sb

eCryptfs wasn't setting s_blocksize in it's superblock; just pick it up
from the lower FS.  Having an s_blocksize of 0 made things like "filefrag"
which call FIGETBSZ unhappy.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Mike Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>