fs/adfs: dir: update directory locking

Update directory locking such that it covers the validation of the
directory, which could fail if another thread is concurrently writing
to the same directory.  Since we may sleep, we need to use a rwsem
rather than a rw spinlock.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>