Convert panic() into KASSERT().

Reviewed by:	alc
MFC after:	1 week
Eliminate the layering violation in the kern_sendfile().  When quering
the file size, use VOP_GETATTR() instead of accessing vnode vm_object
un_pager.vnp.vnp_size.

Take the shared vnode lock earlier to cover the added VOP_GETATTR()
call and, as consequence, the whole internal sendfile loop.  Reduce vm
object lock scope to not protect the local calculations.

Note that this is the last misuse of the vnp_size in the tree, the
others were removed from the ELF image activator by r230246.

Reviewed by:	alc
Tested by:	pho, bf (previous version)
MFC after:	1 week