afs: Fix EEXIST error returned from afs_rmdir() to be ENOTEMPTY

afs: Fix EEXIST error returned from afs_rmdir() to be ENOTEMPTY

AFS servers pass back a code indicating EEXIST when they're asked to remove
a directory that is not empty rather than ENOTEMPTY because not all the
systems that an AFS server can run on have the latter error available and
AFS preexisted the addition of that error in general.

Fix afs_rmdir() to translate EEXIST to ENOTEMPTY.

Fixes: 260a980317da ("[AFS]: Add "directory write" support.")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-13-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>