e2fsck: do not crash on long log file names

e2fsck: do not crash on long log file names

Previously e2fsck would corrupt memory if the log file name was longer
than 100 bytes (e.g., a long log_filename value in e2fsck.conf or a
pattern that expands out to more than 100 bytes).  This was due to
incorrectly calling realloc() in append_string() on the struct string
instead of the malloc'ed char* buffer, among other problems.  This
patch fixes the call to realloc() and also ensures that the buffer is
grown by sufficiently many bytes (not just by 2x).

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>