fs/vfs: Implement EPOLLONESHOT flag

https://linux.die.net/man/2/epoll_ctl:
EPOLLONESHOT (since Linux 2.6.2)
Sets the one-shot behavior for the associated file descriptor.
This means that after an event is pulled out with epoll_wait(2)
the associated file descriptor is internally disabled and
no other events will be reported by the epoll interface.
The user must call epoll_ctl() with EPOLL_CTL_MOD to
rearm the file descriptor with a new event mask.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I6c0dc93e1cdae0e8cea5b487c7005de2da2c2ec3

fs/vfs/fs_epoll.c

@@ -263,6 +263,11 @@ int epoll_pwait(int epfd, FAR struct epoll_event *evs,
         {
           evs[i].data.fd = eph->evs[counter].data.fd;
           evs[i].events  = eph->evs[counter].revents;
+          if (eph->evs[counter].events & EPOLLONESHOT)
+            {
+              eph->evs[counter].events = 0; /* Disable oneshot internally */
+            }
+
           i += 1;
         }
     }

include/sys/epoll.h

@@ -74,6 +74,8 @@ enum EPOLL_EVENTS
 #define EPOLLERR EPOLLERR
     EPOLLHUP = POLLHUP,
 #define EPOLLHUP EPOLLHUP
+    EPOLLONESHOT = 1u << 30,
+#define EPOLLONESHOT EPOLLONESHOT
   };
 
 /* Flags to be passed to epoll_create1.  */