Fix a potential infinite loop in extract_user_to_sg()

Fix extract_user_to_sg() so that it will break out of the loop if
iov_iter_extract_pages() returns 0 rather than looping around forever.

[Note that I've included two fixes lines as the function got moved to a
different file and renamed]

Fixes: 85dd2c8f ("netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator")
Fixes: f5f82cd1 ("Move netfs_extract_iter_to_sg() to lib/scatterlist.c")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: netfs@lists.linux.dev
Link: https://lore.kernel.org/r/1967121.1714034372@warthog.procyon.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>