e2image: fix invalid lseek error detection

e2image: fix invalid lseek error detection

In flush_l2_cache() we are using ext2fs_llseek() however we do not
properly detect the error code returned from the function, because we
are assigning it into ULL variable, hence we will not see negative
values.

Fix this by changing the type of the variable to ext2_loff_t which is
signed and hence will store negative values.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>