f2fs: Fix gcc9 error -Werror=maybe-uninitialized

f2fs: Fix gcc9 error -Werror=maybe-uninitialized

The function grub_get_node_path() could return uninitialized offset with
level == 0 if the block is greater than direct_index + 2 * direct_blks +
2 * indirect_blks + dindirect_blks. The uninitialized offset is then used
by function grub_f2fs_get_block() because level == 0 is valid and
meaningful return to be processed.

The fix is to set level = -1 as return value by grub_get_node_path() to
signify an error that the input block cannot be handled. Any caller
should therefore check level is negative or not before processing the
output.

Reported-by: Neil MacLeod <neil@nmacleod.com>
Signed-off-by: Michael Chang <mchang@suse.com>
Tested-by: Neil MacLeod <neil@nmacleod.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>