cifs: fix another memleak, in cifs_root_iget

cifs_root_iget allocates full_path through
cifs_build_path_to_root, but fails to kfree it upon
cifs_get_inode_info* failure.

Make all failure exit paths traverse clean up
handling at the end of the function.

Signed-off-by: Oskar Schirmer <oskar@scara.com>
Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Cc: stable@kernel.org
Signed-off-by: Steve French <sfrench@us.ibm.com>