xfrm: iptfs: preserve shared-frag marker in iptfs_consume_frags()

iptfs_consume_frags() transfers paged fragments from one socket buffer
to another but fails to propagate the SKBFL_SHARED_FRAG flag. This is
the same class of bug that was fixed in skb_try_coalesce() for
CVE-2026-46300: when fragments backed by read-only page-cache pages are
merged, the marker indicating their shared nature must be preserved so
that ESP can decide correctly whether in-place encryption is safe.

Apply the same two-line fix used in skb_try_coalesce() to
iptfs_consume_frags().

Fixes: b96ba312e21c ("xfrm: iptfs: share page fragments of inner packets")
Cc: stable@vger.kernel.org # 6.14+
Signed-off-by: Takao Sato <takaosato1997@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>