gh-117394: Speed up os.path.ismount() on Posix

[serhiy-storchaka]

It is now 2-3 times faster if the user has permissions.

• Issue: Reduce syscalls for posixpath.ismount #117394

[nineteendo]

Benchmark:

# test.sh
echo "relative" && python -m timeit -s "import before.posixpath" "before.posixpath.ismount('.')" && python -m timeit -s "import after.posixpath" "after.posixpath.ismount('.')"
echo "absolute" && python -m timeit -s "import before.posixpath" "before.posixpath.ismount('/Volumes/2GB_001')" && python -m timeit -s "import after.posixpath" "after.posixpath.ismount('/Volumes/2GB_001')"
echo "no permission" && python -m timeit -s "import before.posixpath" "before.posixpath.ismount('/Users/wannes/path-picker/permission-test/dirs/666')" && python -m timeit -s "import after.posixpath" "after.posixpath.ismount('/Users/wannes/path-picker/permission-test/dirs/666')"

relative
10000 loops, best of 5: 27.3 usec per loop # before
50000 loops, best of 5: 8.25 usec per loop # after
# -> 3.29x faster
absolute
20000 loops, best of 5: 19.7 usec per loop # before
50000 loops, best of 5: 8.21 usec per loop # after
# -> 2.40x faster
no permission
5000 loops, best of 5: 41.2 usec per loop # before
5000 loops, best of 5: 48.5 usec per loop # after
# -> 1.18x slower

[nineteendo]

@erlend-aasland, could you review this?

[erlend-aasland]

Looks good to me, but I'm not sure if it worth the added maintenance cost to optimise this. I mean, how often is the ismount API used? I'll leave it for Serhiy to decide.