ocfs2: Handle errors while setting external xattr values.

ocfs2 can store extended attribute values as large as a single file.  It
does this using a standard ocfs2 btree for the large value.  However,
the previous code did not handle all error cases cleanly.

There are multiple problems to have.

1) We have trouble allocating space for a new xattr.  This leaves us
   with an empty xattr.
2) We overwrote an existing local xattr with a value root, and now we
   have an error allocating the storage.  This leaves us an empty xattr.
   where there used to be a value.  The value is lost.
3) We have trouble truncating a reused value.  This leaves us with the
   original entry pointing to the truncated original value.  The value
   is lost.
4) We have trouble extending the storage on a reused value.  This leaves
   us with the original value safely in place, but with more storage
   allocated when needed.

This doesn't consider storing local xattrs (values that don't require a
btree).  Those only fail when the journal fails.

Case (1) is easy.  We just remove the xattr we added.  We leak the
storage because we can't safely remove it, but otherwise everything is
happy.  We'll print a warning about the leak.

Case (4) is easy.  We still have the original value in place.  We can
just leave the extra storage attached to this xattr.  We return the
error, but the old value is untouched.  We print a warning about the
storage.

Case (2) and (3) are hard because we've lost the original values.  In
the old code, we ended up with values that could be partially read.
That's not good.  Instead, we just wipe the xattr entry and leak the
storage.  It stinks that the original value is lost, but now there isn't
a partial value to be read.  We'll print a big fat warning.

Signed-off-by: Joel Becker <joel.becker@oracle.com>