[PATCH bpf] bpf_doc: use silent mode when exec make cmd

[Hangbin Liu <liuhangbin@xxxxxxxxx>]

When getting kernel version via make, the result may be polluted by other
output, like directory change info. e.g.
$ export MAKEFLAGS="-w"
$ make kernelversion
make: Entering directory '/home/net'
6.8.0
make: Leaving directory '/home/net'

This will distort the reStructuredText output and make latter rst2man
failed like:
bpf-helpers.rst:20: (WARNING/2) Field list ends without a blank line; unexpected unindent.

Using silent mode would help. e.g.
$ make -s --no-print-directory kernelversion
6.8.0

Fixes: fd0a38f9c37d ("scripts/bpf: Set version attribute for bpf-helpers(7) man page")
Signed-off-by: Michael Hofmann <mhofmann@xxxxxxxxxx>
Signed-off-by: Hangbin Liu <liuhangbin@xxxxxxxxx>
---
 scripts/bpf_doc.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index 4606944984ee..c55878bddfdd 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -414,8 +414,8 @@ class PrinterRST(Printer):
             version = version.stdout.decode().rstrip()
         except:
             try:
-                version = subprocess.run(['make', 'kernelversion'], cwd=linuxRoot,
-                                         capture_output=True, check=True)
+                version = subprocess.run(['make', '-s', '--no-print-directory', 'kernelversion'],
+                                         cwd=linuxRoot, capture_output=True, check=True)
                 version = version.stdout.decode().rstrip()
             except:
                 return 'Linux'
-- 
2.43.0