jffs2: fix unbalanced locking

Li Zefan reported an unbalanced locking issue, found by his
internal debugging feature on runtime. The particular case he was
looking at doesn't lead to a deadlock, as the structure that this lock
is embedded in is freed on error. But we should straighten out the error
handling.

Because several callers of jffs2_do_read_inode_internal() /
jffs2_do_read_inode() already handle the locking/unlocking and inode
clearing at their own level, let's just push any unlocks/clearing down
to the caller. This consistency is much easier to verify.

Reported-by: Li Zefan <lizefan@huawei.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>