nfsd: Define the file access mode enum for tracing

[ Upstream commit c19285596de699e4602f9c89785e6b8c29422286 ]

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Stable-dep-of: 3bc8edc98bd4 ("nfsd: under NFSv4.1, fix double svc_xprt_put on rpc_create failure")
Signed-off-by: Sasha Levin <sashal@kernel.org>