fs: split off do_getxattr from getxattr

This splits off do_getxattr function from the getxattr function. This will
allow io_uring to call it from its io worker.

Signed-off-by: Stefan Roesch <shr@fb.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20220323154420.3301504-3-shr@fb.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>