f2fs: Add sanity_check_inode() function

This was done as part of commits 5d64600d4f33 "f2fs: avoid bug_on on
corrupted inode" and 76d56d4ab4f2 "f2fs: fix to do sanity check with
extra_attr feature" upstream, but the specific checks they added are
not applicable to 4.9.

Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>