jffs2: use mutex_is_locked() in __jffs2_flush_wbuf()

Use a helper to test if a mutex is held instead of a hack with
mutex_trylock().

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>