Fix emulation of the fcntl64() syscall. In Linux, this is exactly
the same as fcntl() except that it supports the new 64-bit file
locking commands (LINUX_F_GETLK64 etc) that use the `flock64'
structure. We had been interpreting all flock structures passed to
fcntl64() as `struct flock64' instead of only the ones from F_*64
commands.

The glibc in linux_base-7 uses fcntl64() by default, but the bug
was often non-fatal since the misinterpretation typically only
causes junk to appear in the `l_len' field and most junk values are
accepted as valid range lengths. The result is occasional EINVAL
errors from F_SETLK and a few bytes after the supplied `struct
flock' getting clobbered during F_GETLK.

PR: kern/37656
Reviewed by: marcel
Approved by: re
MFC after: 1 week