In fhopen, vfs_ref() the mount point while vnode is unlocked, to prevent vn_start_write(NULL, &mp) from operating on potentially freed or reused struct mount *.
In fhopen, vfs_ref() the mount point while vnode is unlocked, to prevent
vn_start_write(NULL, &mp) from operating on potentially freed or reused
struct mount *.

Remove unmatched vfs_rel() in cleanup.

Noted and reviewed by:	tegge
Tested by:	pho
MFC after:	3 days

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