jffs2: Fix page lock / f->sem deadlock

commit 49e91e7079febe59a20ca885a87dd1c54240d0f1 upstream.

With this fix, all code paths should now be obtaining the page lock before
f->sem.

Reported-by: Szabó Tamás <sztomi89@gmail.com>
Tested-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>