jffs2: fix up error handling for insert_inode_locked

jffs2: fix up error handling for insert_inode_locked

after 250df6ed274d767da844a5d9f05720b804240197
(fs: protect inode->i_state with inode->i_lock), insert_inode_locked()
no longer returns the inode with I_NEW set on failure.  However,
the error handler still calls unlock_new_inode() on failure,
which does a WARN_ON if I_NEW is not set, so any failure spews
a lot of warnings.

We can just drop the unlock_new_inode() if insert_inode_locked()
fails here.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>