[CIFS] remove bkl usage from umount begin

The lock_kernel call moved into the fs for umount_begin
is not needed.  This adds a check to make sure we don't
call umount_begin twice on the same fs.

umount_begin for cifs is probably not needed and
may eventually be able to be removed, but in
the meantime this smaller patch is safe and
gets rid of the bkl from this path which provides
some benefit.

Acked-by: Jeff Layton <redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>