listxattrat: READ-classify return for xattr-list family consistency

listxattrat(2) (Linux 6.13+) returns the size in bytes of the list of
extended attribute names, exactly like listxattr/llistxattr/flistxattr,
but its exit was classified UNCLASSIFIED, so its read bytes were dropped
from I/O totals. Classify it as ReadClassified and regenerate the BPF
handler (ret_type now READ_CLASSIFIED). This mirrors the getxattrat fix
(task ku, commit c3177bd) and completes xattr-family consistency:
get-family and list-family are READ_CLASSIFIED while set-family and
remove-family stay UNCLASSIFIED (they return 0/-1).

Update the docs ReadClassified list and the retclassify expectation, and
add an ioworkload scenario plus integration test: the workload sets a
user xattr then lists names via the raw listxattrat(2) syscall with
AT_FDCWD, and the test asserts enter_listxattrat captures the file path
and accounts the returned name-list size as read bytes.

Task: r20

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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