bcachefs: Annotate struct bch_xattr with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
x_name to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>