net: stmmac: fix TX descriptor availability check for TSO traffic

stmmac_tso_xmit() estimates the number of free TX descriptors required by
a TSO skb as:

	(skb->len - proto_hdr_len) / TSO_MAX_BUFF_SIZE + 1

which assumes the payload is split into TSO_MAX_BUFF_SIZE chunks. This
underestimates the descriptors actually consumed by stmmac_tso_allocator(),
since each fragment is mapped individually and so it needs at least one
descriptor regardless of its size. Moreover, one descriptor is used for
the L2/L3/L4 headers and, when the MSS changes, one more is consumed for
the MSS context descriptor.
For a highly fragmented TSO skb the check can therefore pass even when the
ring has too few free slots. stmmac_tso_allocator() then writes past the
available descriptors, overwriting descriptors still owned by the DMA
engine, corrupting the TX ring.
Add stmmac_tso_get_num_desc() to compute the exact number of descriptors
needed for the header, the linear payload and each fragment, plus the MSS
context descriptor when required, and use it in the availability check.

Fixes: f748be531d70 ("stmmac: support new GMAC4")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
Link: https://patch.msgid.link/20260907-stmmac-fix-tso-nfrags-check-v1-1-328459906cdb@oss.qualcomm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>