Fix non-printable characters in NVMe model and serial numbers.

The NVMe 1.4 spec simply says that Model and Serial numbers are
ASCII strings. Unlike SCSI, it doesn't prohibit non-printable
characters or say that the strings should be padded with spaces.

Since 2014, we have had cam_strvis_sbuf(), which gives additional
options for handling non-ASCII characters. That behavior hasn't
been available for non-sbuf consumers, so users of cam_strvis()
were left with having octal ASCII codes inserted.

So, to avoid having garbage or octal chracters in the strings, use
cam_strvis_sbuf() to create a new function, cam_strvis_flag(), and
re-implement cam_strvis() using cam_strvis_flag().

Now, for the NVMe drives, we can use cam_strvis_flag with the
CAM_STRVIS_FLAG_NONASCII_SPC flag. This transforms non-printable
characters into spaces.

sys/cam/cam.c:
Add a new function, cam_strvis_flag(), that creates an sbuf
on the stack with the user's destination buffer, and calls
cam_strvis_sbuf() with the given flag argument.

Re-implement cam_strvis() to call cam_strvis_flag with the
CAM_STRVIS_FLAG_NONASCII_ESC argument. This should be the
equivalent of the old cam_strvis() function, except for the
overhead of creating the sbuf and calling sbuf_putc/printf.

sys/cam/cam.h:
Declaration for cam_strvis_flag.

sys/cam/nvme/nvme_all.c:
In nvme_print_ident, use the NONASCII_SPC flag with
cam_strvis_flag().

sys/cam/nvme/nvme_da.c:
In ndaregister(), use cam_strvis_flag() with the
NONASCII_SPC flag for the disk description and serial
number we report to GEOM.

sys/cam/nvme/nvme_xpt.c:
In nvme_probe_done(), use cam_strvis_flag with the
NONASCII_SPC flag when storing the drive serial number
in the CAM EDT.

MFC after: 1 week
Sponsored by: Spectra Logic
Differential Revision: https://reviews.freebsd.org/D33973