cifs: clean up variables in cifs_unlink

Change parameters to cifs_unlink to match the ones used in the generic
VFS. Add some local variables to cut down on the amount of struct
dereferencing that needs to be done, and eliminate some unneeded NULL
pointer checks on the parent directory inode. Finally, rename pTcon
to "tcon" to more closely match standard kernel coding style.

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