ext2fs: fix warning: 'blocknxt' may be used uninitialized

ext2fs: fix warning: 'blocknxt' may be used uninitialized

This warning was introduced in 436da3c "ext2load: increase read
speed":

ext2fs.c: In function 'ext2fs_read_file':
ext2fs.c:458:19: warning: 'blocknxt' may be used uninitialized in this function [-Wuninitialized]

this change makes it go away.

Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Jason Cooper <u-boot@lakedaemon.net>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>