net/smc: cap allocation order for SMC-R physically contiguous buffers

net/smc: cap allocation order for SMC-R physically contiguous buffers

The alloc_pages() cannot satisfy requests exceeding MAX_PAGE_ORDER,
and attempting such allocations will lead to guaranteed failures
and potential kernel warnings.

For SMCR_PHYS_CONT_BUFS, cap the allocation order to MAX_PAGE_ORDER.
This ensures the attempts to allocate the largest possible physically
contiguous chunk succeed, instead of failing with an invalid order.
This also avoids redundant "try-fail-degrade" cycles in
__smc_buf_create().

For SMCR_MIXED_BUFS, no cap is needed: if the order exceeds
MAX_PAGE_ORDER, alloc_pages() will silently fail (__GFP_NOWARN)
and automatically fall back to virtual memory.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Sidraya Jayagond <sidraya@linux.ibm.com>
Link: https://patch.msgid.link/20260429021637.21815-1-alibuda@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>