UBIFS: fix crash on error path

This patch fixes a regression introduced by
"49942976 UBIFS: make ubifs_lpt_init clean-up in case of failure" which
I've hit while running the 'integck -p' test. When remount the file-system
from R/O mode to R/W mode and 'lpt_init_wr()' fails, we free _all_ LPT
resources by calling 'ubifs_lpt_free(c, 0)', even those needed for R/O
mode. This leads to subsequent crashes, e.g., if we try to unmount
the file-system.

Cc: stable@vger.kernel.org [v3.5+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>