vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

The newly used helper also checks for empty ("") paths.

NULL paths with any flag value other than AT_EMPTY_PATH go the usual
route and end up with -EFAULT to retain compatibility (Rust is abusing
calls of the sort to detect availability of statx).

This avoids path lookup code, lockref management, memory allocation and
in case of NULL path userspace memory access (which can be quite
expensive with SMAP on x86_64).

Benchmarked with statx(..., AT_EMPTY_PATH, ...) running on Sapphire
Rapids, with the "" path for the first two cases and NULL for the last
one.

Results in ops/s:
stock:     4231237
pre-check: 5944063 (+40%)
NULL path: 6601619 (+11%/+56%)

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240625151807.620812-1-mjguzik@gmail.com


Tested-by: Xi Ruoyao <xry111@xry111.site>
[brauner: use path_mounted() and other tweaks]
Signed-off-by: Christian Brauner <brauner@kernel.org>