vfs_vnops.c: Use va_bytes >= va_size hint to avoid SEEK_DATA/SEEKHOLE

vn_generic_copy_file_range() tries to maintain holes
in file ranges being copied, using SEEK_DATA/SEEK_HOLE
where possible,

Unfortunately SEEK_DATA/SEEK_HOLE operations can take
a long time under certain circumstances.
Although it is not currently possible to know if a file has
unallocated data regions, the case where va_bytes >= va_size
is a strong hint that there are no unallocated data regions.
This hint does not work well for file systems doing compression,
but since it is only a hint, it is still useful.

For the case of va_bytes >= va_size, avoid doing SEEK_DATA/SEEK_HOLE.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44509