ext4: remove redundant assignment to node

Pointer 'node' is assigned a value that is never read, node is
later overwritten when it re-assigned a different value inside
the while-loop.  The assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>