NFS: make sure open context mode have FMODE_EXEC when file open for exec

Because file f_mode never have FMODE_EXEC, open context mode won't get
FMODE_EXEC from file f_mode. Open context mode only care about FMODE_READ/
FMODE_WRITE/FMODE_EXEC, and all info about open context mode can be convert
from file f_flags, so convert file f_flags to open context mode by
flags_to_mode().

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