xfs_scrub: fix buffer overflow in string_escape

xfs_scrub: fix buffer overflow in string_escape

Need to allocate one more byte for the null terminator, just in case the
/entire/ input string consists of non-printable bytes e.g. emoji.

Cc: <linux-xfs@vger.kernel.org> # v4.15.0
Fixes: 396cd0223598bb ("xfs_scrub: warn about suspicious characters in directory/xattr names")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>