f2fs: xattr simplifications

f2fs: xattr simplifications

Now that the xattr handler is passed to the xattr handler operations, we
have access to the attribute name prefix, so simplify
f2fs_xattr_generic_list.

Also, f2fs_xattr_advise_list is only ever called for
f2fs_xattr_advise_handler; there is no need to double check for that.

Signed-off-by: Andreas Gruenbacher <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: Changman Lee <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: [email protected]
Signed-off-by: Al Viro <[email protected]>