[JFFS2] Fix return value from jffs2_write_end()

jffs2_write_end() is sometimes passing back a "written" length greater 
than the length we passed into it, leading to a BUG at mm/filemap.c:1749 
when used with unionfs.

It happens because we actually write more than was requested, to reduce 
log fragmentation. These "longer" writes are fine, but they shouldn't 
get propagated back to the vm/vfs.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>