smb: client: fix perf regression with deferred closes

Customer reported that one of their applications started failing to
open files with STATUS_INSUFFICIENT_RESOURCES due to NetApp server
hitting the maximum number of opens to same file that it would allow
for a single client connection.

It turned out the client was failing to reuse open handles with
deferred closes because matching ->f_flags directly without masking
off O_CREAT|O_EXCL|O_TRUNC bits first broke the comparision and then
client ended up with thousands of deferred closes to same file.  Those
bits are already satisfied on the original open, so no need to check
them against existing open handles.

Reproducer:

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <pthread.h>

 #define NR_THREADS      4
 #define NR_ITERATIONS   2500
 #define TEST_FILE       "/mnt/1/test/dir/foo"

 static char buf[64];

 static void *worker(void *arg)
 {
         int i, j;
         int fd;

         for (i = 0; i < NR_ITERATIONS; i++) {
                 fd = open(TEST_FILE, O_WRONLY|O_CREAT|O_APPEND, 0666);
                 for (j = 0; j < 16; j++)
                         write(fd, buf, sizeof(buf));
                 close(fd);
         }
 }

 int main(int argc, char *argv[])
 {
         pthread_t t[NR_THREADS];
         int fd;
         int i;

         fd = open(TEST_FILE, O_WRONLY|O_CREAT|O_TRUNC, 0666);
         close(fd);
         memset(buf, 'a', sizeof(buf));
         for (i = 0; i < NR_THREADS; i++)
                 pthread_create(&t[i], NULL, worker, NULL);
         for (i = 0; i < NR_THREADS; i++)
                 pthread_join(t[i], NULL);
         return 0;
 }

Before patch:

$ mount.cifs //srv/share /mnt/1 -o ...
$ mkdir -p /mnt/1/test/dir
$ gcc repro.c && ./a.out
...
number of opens: 1391

After patch:

$ mount.cifs //srv/share /mnt/1 -o ...
$ mkdir -p /mnt/1/test/dir
$ gcc repro.c && ./a.out
...
number of opens: 1

Cc: linux-cifs@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: Jay Shin <jaeshin@redhat.com>
Cc: Pierguido Lambri <plambri@redhat.com>
Fixes: b8ea3b1f ("smb: enable reuse of deferred file handles for write operations")
Acked-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: Steve French <stfrench@microsoft.com>