fs/f2fs: Do not read past the end of nat journal entries

A corrupt f2fs file system could specify a nat journal entry count
that is beyond the maximum NAT_JOURNAL_ENTRIES.

Check if the specified nat journal entry count before accessing the
array, and throw an error if it is too large.

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>