ovl: get rid of redundant members in struct ovl_fs

ofs->upper_mnt is copied to ->layers[0].mnt and ->layers[0].trap could be
used instead of a separate ->upperdir_trap.

Split the lowerdir option early to get the number of layers, then allocate
the ->layers array, and finally fill the upper and lower layers, as before.

Get rid of path_put_init() in ovl_lower_dir(), since the only caller will
take care of that.

[Colin Ian King] Fix null pointer dereference on null stack pointer on
error return found by Coverity.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>