NFSv4: check FMODE_EXEC from open context mode in nfs4_opendata_access()

After converting file f_flags to open context mode by flags_to_mode(), open
context mode will have FMODE_EXEC when file open for exec, so we check
FMODE_EXEC from open context mode.

No functional change, just simplify the code.

Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>