Use uint64 sysctl when retrieving vfs.bufspace on FreeBSD

[danielnelson]

On some system using a the uint32 function returns the error "cannot allocate memory". influxdata/telegraf#3750

It appears that the size of this field can vary, and sometimes requires additional space. From man 3 sysctl:

The information is copied into the buffer specified by oldp. The size of
the buffer is given by the location specified by oldlenp before the call,
and that location gives the amount of data copied after a successful call
and after a call that returns with the error code ENOMEM. If the amount
of data available is greater than the size of the buffer supplied, the
call supplies as much data as fits in the buffer provided and returns
with the error code ENOMEM. If the old value is not desired, oldp and
oldlenp should be set to NULL.

It is possible that we should use func SysctlRaw instead as it checks the size of the field using the method described here:

The size of the available data can be determined by calling sysctl() with
the NULL argument for oldp. The size of the available data will be
returned in the location pointed to by oldlenp. For some operations, the
amount of space may change often. For these operations, the system
attempts to round up so that the returned size is large enough for a call
to return the data shortly thereafter.

I have not made this larger change.

[shirou]

Thanks a lot! I confirmed this works on my FreeBSD environment.