jffs2:jffs2_1pass.c: remove double braces

Clang interpretes an if condition like  "if ((a = b) == NULL)
as it tries to assign a value in a statement. Hence if you do
"if ((something)) it warns you that you might be confused.
Hence drop the double braces for plane if statements.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>