ext2: xip check fix

ext2 should not worry about checking sb->s_blocksize for XIP before the
sb's blocksize actually gets set.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>