xfstests: fix up fs_perms test used by 126

xfstests: fix up fs_perms test used by 126

Test 126 was failing intermittently for Ted & I; it seems that
this is because we were passing an unterminated string to
fopen for the mode; I'm not certain why this made it fail,
but it's pretty clearly not a good thing to do, and fixing
it fixes the test.

Rather than passing around characters, do things string-wise,
since that is what is ultimately used in fopen().

Reported-by: Theodore Tso <tytso@mit.edu>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>