jffs2: allow to discriminate between recoverable and non-recoverable errors

This patch is basically a revert of commit f326966b.

It allows JFFS2 to make the distinction between a potential transient
error (reading or writing the media) and a non recoverable error like a
bad CRC on the extended attribute data or some insconsitent parameters.

In order to make clear that the error is indeed intended to report a
corrupted attribute, a new local error code (JFFS2_XATTR_IS_CORRUPTED)
is introduced rather than returning a confusing positive EIO, which is
what led to the inappropriate "fix" last time.

This error code is never reported to user space and only checked locally
in this file.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>