VOP_COPY_FILE_RANGE: Add a COPY_FILE_RANGE_TIMEO1SEC flag

Although it is not specified in the RFCs, the concept that
the NFSv4 server should reply to an RPC request within a
reasonable time is accepted practice within the NFSv4 community.

Without this patch, the NFSv4.2 server attempts to reply to
a Copy operation within 1second by limiting the copy to
vfs.nfs.maxcopyrange bytes (default 10Mbytes). This is crude at
best, given the large variation in I/O subsystem performance.

This patch adds a kernel only flag COPY_FILE_RANGE_TIMEO1SEC
that the NFSv4.2 can specify, which tells VOP_COPY_FILE_RANGE()
to return after approximately 1 second with a partial result and
implements this in vn_generic_copy_file_range(), used by
vop_stdcopyfilerange().

Modifying the NFSv4.2 server to set this flag will be done in
a separate patch. Also under consideration is exposing the
COPY_FILE_RANGE_TIMEO1SEC to userland for use on the FreeBSD
copy_file_range(2) syscall.

MFC after: 2 weeks
Reviewed by: khng
Differential Revision: https://reviews.freebsd.org/D31829