debugfs: avoid ambiguity when printing filenames

debugfs: avoid ambiguity when printing filenames

The way debugfs escapes filenames is ambiguous because a sequence like
M-A can mean either the byte 'A' + 128 == 0xc1 or the three bytes
{'M', '-', 'A'}.  Similarly, ^A can mean either the byte
'A' ^ 0x40 == 0x01 or the two bytes {'^', 'A'}.

Fix this and simplify the code by switching to a simpler strategy where
all bytes < 32, all bytes >= 127, and backslash are encoded with C-style
hex escape sequences.  E.g., the byte 0xc1 will now be encoded as \xc1
rather than M-A as it was before, while a filename consisting of the
three bytes {'M', '-', 'A'} will continue to be shown as M-A.

I want to fix this mainly because I want to use debugfs to retrieve raw
encrypted filenames for ciphertext verification tests.  But this doesn't
work if the returned filenames are ambiguous.

Fixes: 68a1de3df340 ("debugfs: pretty print encrypted filenames in the ls command")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>