fuse_vnop_copy_file_range(): add safety

v_mount for unlocked vnode could be NULL, check for it. Explain why it
is safe to access fs-specific data for mp if it is read as non-NULL.

Reviewed by: asomers, jah
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42625