When deallocating the vm object in elf_map_insert() due to
vm_map_insert() failure, drop the vnode lock around the call to
vm_object_deallocate().

Since the deallocated object is the vm object of the vnode, we might
get the vnode lock recursion there. In fact, it is almost impossible
to make vm_map_insert() failing there on stock kernel.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks