a "cylinder group" is a metadata structure in FFS that describes the
allocation state of a portion of the blocks and inodes of the file system
and contains the inode records themselves. the header for this structure
also contains a "magic number" field that is supposed to contain a certain
constant value as a way to sanity-check that this metadata on disk was not
overwritten with some completely unrelated contents.
in your case, since the magic number field does not actually contain the value
that it's supposed to contain, we know that the storage underneath the
file system has gotten corrupted somehow. you'll want to track down
how that happened, but that is separate from your immediate problem.
this sounds like a bug I have seen before, where the extended attribute block
for a file has been corrupted. please try the attached patch and see if
this prevents the infinite loop.
if that does prevent the infinite loop, then the file will probably appear
not to have an ACL anymore, and I'm not sure what will happen if you try
to set a new ACL on the file when it is in this state. for right now,
the safest thing you can do will be to make a copy of the file without
trying to preserve extended attributes (ie. do not use cp's "-p" option),
then delete the original file, then move the copy of the file to have
the original file's name, then you can change the new file's
owner/group/mode/ACL to be what the original file had.