selftests: tls: size splice_short pipe by page size

selftests: tls: size splice_short pipe by page size

splice_short grows its pipe with (MAX_FRAGS + 1) * 0x1000 so it can
queue one short vmsplice() buffer for each fragment before draining the
pipe. That assumes 4K pipe buffers.

On 64K-page kernels the request is rounded to 262144 bytes, which
provides only four pipe buffers. The fifth one-byte vmsplice() blocks in
pipe_wait_writable and the test times out before it reaches the TLS path.

Request enough bytes for the same number of pipe buffers using the
runtime page size, and assert that the kernel granted at least that much.
If an unprivileged run cannot raise the pipe above the system
pipe-max-size limit, skip the test because it cannot exercise the
intended path.

Fixes: 3667e9b442b9 ("selftests: tls: add test for short splice due to full skmsg")
Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260624134416.3235403-1-nirmoyd@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>