selftests: bpf: introduce a common routine for reading procfs

selftests: bpf: introduce a common routine for reading procfs

Parametrize current way of getting MAX_SKB_FRAGS value from {sys,proc}fs
so that it can be re-used to get cache line size of system's CPU. All
that just to mimic and compute size of kernel's struct skb_shared_info
which for xsk and test suite interpret as tailroom.

Introduce two variables to ifobject struct that will carry count of skb
frags and tailroom size. Do the reading and computing once, at the
beginning of test suite execution in xskxceiver, but for test_progs such
way is not possible as in this environment each test setups and torns
down ifobject structs.

Reviewed-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://patch.msgid.link/20260402154958.562179-6-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>