e2fsck: when mutating file name make sure its length never exceeds 255

e2fsck: when mutating file name make sure its length never exceeds 255

E2fsck will attempt to mutate filenames to ensure uniqueness if
necessary.  If there are two unique filenames that are 254 or 255
characters in length and do not contain the '~' character, the
mutate_name() function would create a filename which is 256 bytes
long, which is not a legal filename in Linux.  Adjust the mutate_name
function to avoid this possibility.

Addresses-Coverity-Bug: 1500768
Signed-off-by: Theodore Ts'o <tytso@mit.edu>