fuse: Return EPERM rather than ENOSYS from link()

[ Upstream commit 83442135 ]

link() is documented to return EPERM when a filesystem doesn't support
the operation, return that instead.

Link: https://github.com/libfuse/libfuse/issues/925


Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>