[JFFS2] Fix race in setting file attributes

It seems like there is a potential race in the function jffs2_do_setattr()
in the case when attributes of a symlink are updated. The symlink metadata
is read without having f->sem locked.

The following patch should fix the race.

Signed-off-by: Dmitry Bazhenov <atrey@emcraft.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>