smb: client: reduce stack usage in cifs_try_adding_channels()

Clang warns about exceeded stack frame size

  fs/smb/client/sess.c:160:5: warning: stack frame size (1368) exceeds
  limit (1024) in 'cifs_try_adding_channels' [-Wframe-larger-than]

It turns out that cifs_ses_add_channel() got inlined into
cifs_try_adding_channels() which had a stack-allocated variable @ctx
of 624 bytes in size.  Fix this by making it heap-allocated.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202307270640.5ODmPwDl-lkp@intel.com/
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>