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 <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Jason Cooper <[email protected]>
Cc: Andreas Bießmann <[email protected]>
Cc: Reinhard Arlt <[email protected]>
Signed-off-by: Kim Phillips <[email protected]>