[BUG] fs/dcache: might_sleep is called under a spinlock

[Jia-Ju Bai]

According to fs/dcache.c, might_sleep is called under a spinlock,
and the function call path is:
d_prune_aliases (acquire the spinlock)
  dput
    might_sleep

This bug is found by my static analysis tool and my code review.
A possible fix is to remove might_sleep in dput.

Thanks,
Jia-Ju Bai