[JFFS2] Account summary space in reserved_size.

Always keep valid data in reserved_size.

It did not cause problems, but the reservation code was unoptimal
when centralized summary was active or the size of the erase block
was very small.

Signed-off-by: Ferenc Havasi <havasi@inf.u-szeged.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>