cifs: split out dfs code from cifs_reconnect()

cifs: split out dfs code from cifs_reconnect()

[ Upstream commit bbcce368044572d0802c3bbb8ef3fe98f581d803 ]

Make two separate functions that handle dfs and non-dfs reconnect
logics since cifs_reconnect() became way too complex to handle both.
While at it, add some documentation.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: 1dcdf5f5b213 ("cifs: Fix connections leak when tlink setup failed")
Signed-off-by: Sasha Levin <sashal@kernel.org>