msdosfs: fix bug in msfdosfs_hashins().

msdosfs: fix bug in msfdosfs_hashins().

This function returns an error if there is already a denode in the hash table:
EBUSY if the hashed denode represents a live file and EINVAL if it represents a
deleted but still opened file.

There was a typo in the function causing it to check for liveness in the denode
to insert instead of the already inserted one.  As a consequence, if N threads
were in a race in deget() to insert a new denode for the same file in the hash
table, the losers would fail with EINVAL instead of retrying.