NFSv4: Handle open for execute correctly

When mapping the NFSv4 context to an open mode and access mode,
we need to treat the FMODE_EXEC flag differently. For the open
mode, FMODE_EXEC means we need read share access. For the access
mode checking, we need to verify that the user actually has
execute access.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>