fs/f2fs: Do not copy file names that are too long

fs/f2fs: Do not copy file names that are too long

A corrupt f2fs file system might specify a name length which is greater
than the maximum name length supported by the GRUB f2fs driver.

We will allocate enough memory to store the overly long name, but there
are only F2FS_NAME_LEN bytes in the source, so we would read past the end
of the source.

While checking directory entries, do not copy a file name with an invalid
length.

Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>