vfs: Distinguish between full xattr names and proper prefixes

vfs: Distinguish between full xattr names and proper prefixes

Add an additional "name" field to struct xattr_handler.  When the name
is set, the handler matches attributes with exactly that name.  When the
prefix is set instead, the handler matches attributes with the given
prefix and with a non-empty suffix.

This patch should avoid bugs like the one fixed in commit c361016a in
the future.

Signed-off-by: Andreas Gruenbacher <[email protected]>
Reviewed-by: James Morris <[email protected]>
Signed-off-by: Al Viro <[email protected]>