bcachefs: Remove incorrect __counted_by annotation

This actually reverts 86e92eee ("bcachefs: Annotate struct bch_xattr
with __counted_by()").

After the x_name, there is a value. According to the disscussion[1],
__counted_by assumes that the flexible array member contains exactly
the amount of elements that are specified. Now there are users came across
a false positive detection of an out of bounds write caused by
the __counted_by here[2], so revert that.

[1] https://lore.kernel.org/lkml/Zv8VDKWN1GzLRT-_@archlinux/T/#m0ce9541c5070146320efd4f928cc1ff8de69e9b2
[2] https://privatebin.net/?a0d4e97d590d71e1#9bLmp2Kb5NU6X6cZEucchDcu88HzUQwHUah8okKPReEt



Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>