ovl: Switch to generic_removexattr

Commit d837a49b ("ovl: fix POSIX ACL setting") switches from
iop->setxattr from ovl_setxattr to generic_setxattr, so switch from
ovl_removexattr to generic_removexattr as well.  As far as permission
checking goes, the same rules should apply in either case.

While doing that, rename ovl_setxattr to ovl_xattr_set to indicate that
this is not an iop->setxattr implementation and remove the unused inode
argument.

Move ovl_other_xattr_set above ovl_own_xattr_set so that they match the
order of handlers in ovl_xattr_handlers.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Fixes: d837a49b ("ovl: fix POSIX ACL setting")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>