[JFFS2] fix deadlock on error path

When the MTD driver returns write failure, the following deadlock
occurs:

We are in __jffs2_flush_wbuf(), we hold &c->wbuf_sem. Write failure.
jffs2_wbuf_recover()->jffs2_reserve_space_gc()->jffs2_do_reserve_space()
->jffs2_erase_pending_blocks()->jffs2_flash_read()

and it tries to lock &c->wbuf_sem again. Deadlock.

Reported-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>