autofs - Fix sparse warning: context imbalance in autofs4_d_automount() different lock contexts for basic block

Sparse complains:

  fs/autofs4/root.c:409:9: sparse: context imbalance in 'autofs4_d_automount' - different lock contexts for basic block

This was introduced by commit f55fb0c2 ("autofs4 - dont clear
DCACHE_NEED_AUTOMOUNT on rootless mount")

The function autofs4_d_automount can be left with the (&sbi->fs_lock)
held if sbi->version <= 4 and simple_empty(dentry) == false so the
warning seems valid.

--> Add an spin_unlock in this case before we jump to done

Unfortunately compile tested only.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NPeter Huewe <peterhuewe@gmx.de>
Acked-by: NIan Kent <raven@themaw.net>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>