net/sched: cls_flower: validate mask pointer after nla_next()

In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_flower: validate mask pointer after nla_next()

fl_set_enc_opt() iterates the key's nested tunnel-option attributes
with nla_for_each_attr() while advancing a single mask pointer via
nla_next() at the bottom of each loop, so the mask cursor is driven
by the number of key attributes rather than by the mask's own
attributes. The nla_ok() added by commit c96adff956191 ("cls_flower:
call nla_ok() before nla_next()") only validates the mask pointer
that was just consumed; the pointer produced by nla_next() is used by
the next iteration (fl_set_geneve_opt() and siblings) without any
validation.

The mask's nested attributes are validated with NL_VALIDATE_LIBERAL,
which merely warns on trailing bytes that do not form a complete
attribute. A mask carrying one valid attribute plus 1-3 residue
bytes (or a non-aligned attribute length making msk_depth negative)
therefore reaches the next iteration with msk_depth != 0, so neither
the !msk_depth check in fl_set_enc_opt() nor the !depth check in the
per-type helpers fires. nla_type() then reads past the mask payload
and nla_parse_nested_deprecated() iterates with an nla_len taken
from those bytes, reading well beyond the mask attribute (KASAN:
slab-out-of-bounds read in __nla_validate_parse from fl_change()).

Validate the advanced mask pointer as well: when the mask is not
legitimately exhausted (msk_depth != 0) and the new pointer fails
nla_ok(), reject the filter with -EINVAL. An exactly exhausted mask
still skips the check, preserving exact-match behaviour for the
remaining key attributes.