fs: split off setxattr_copy and do_setxattr function from setxattr

fs: split off setxattr_copy and do_setxattr function from setxattr

This splits of the setup part of the function
setxattr in its own dedicated function called
setxattr_copy. In addition it also exposes a
new function called do_setxattr for making the
setxattr call.

This makes it possible to call these two functions
from io_uring in the processing of an xattr request.

Signed-off-by: Stefan Roesch <shr@fb.com>
Link: https://lore.kernel.org/r/20211223235123.4092764-3-shr@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>