ext2fs: fix warning: 'blocknxt' may be used uninitialized with gcc 4.2

ext2fs: fix warning: 'blocknxt' may be used uninitialized with gcc 4.2

The above warning was introduced originally in 436da3c "ext2load:
increase read speed" and fixed for newer toolchains in b803273 "ext2fs:
fix warning: 'blocknxt' may be used uninitialized".  This change did not
fix the warning with gcc 4.2, as found in ELDK 4.2.

If we rework the while loop to initalize blocknxt before entering the
warning really goes away.  Tested on am335x with an approx 7mb file and
crc32 in U-Boot befor and after this change.

Cc: Wolfgang Denk <wd@denx.de>
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>
Cc: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Tom Rini <trini@ti.com>