xfs_scrub: report deceptive file extensions

xfs_scrub: report deceptive file extensions

Earlier this year, ESET revealed that Linux users had been tricked into
opening executables containing malware payloads.  The trickery came in
the form of a malicious zip file containing a filename with the string
"job offer․pdf".  Note that the filename does *not* denote a real pdf
file, since the last four codepoints in the file name are "ONE DOT
LEADER", p, d, and f.  Not period (ok, FULL STOP), p, d, f like you'd
normally expect.

Teach xfs_scrub to look for codepoints that could be confused with a
period followed by alphanumerics.

Link: https://www.welivesecurity.com/2023/04/20/linux-malware-strengthens-links-lazarus-3cx-supply-chain-attack/
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>