NFSv4: Clear the NFS_CAP_XATTR flag if not supported by the server

[ Upstream commit 4fb2b677 ]

nfs_server_set_fsinfo() shouldn't assume that NFS_CAP_XATTR is unset
on entry to the function.

Fixes: b78ef845 ("NFSv4.2: query the server for extended attribute support")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>